Method for producing a multicoat paint system

ABSTRACT

The present invention relates to a method for producing a multicoat paint system on a metallic substrate, in which a basecoat or a plurality of directly successive basecoats are produced directly on a metallic substrate coated with a cured electrocoat, a clearcoat is produced directly on the one basecoat or the uppermost of the plurality of basecoats, and then the one or more basecoats and the clearcoat are jointly cured, and wherein at least one basecoat material used for production of the basecoats comprises the following components: an aqueous dispersion comprising at least one specific copolymer, a specific linear hydroxy-functional reaction product and a polyurethane resin, the preparation of which involves using at least one compound containing at least one carboxylic acid group and at least one group reactive toward isocyanate groups.

The present invention relates to a method for producing a multicoatpaint system, in which a basecoat or a plurality of directly successivebasecoats are produced directly on a metallic substrate coated with acured electrocoat, a clearcoat is produced directly on the one basecoator the uppermost of the plurality of basecoats, and then the one or morebasecoats and the clearcoat are jointly cured. The present inventionadditionally relates to a multicoat paint system which has been producedby the method of the invention.

PRIOR ART

Multicoat paint systems on metallic substrates, for example multicoatpaint systems in the automobile industry, are known. In general,multicoat paint systems of this kind comprise, viewed from the metallicsubstrate outward, an electrocoat, a layer which has been applieddirectly to the electrocoat and is usually referred to as theprimer-surfacer coat, at least one coat which comprises color pigmentsand/or effect pigments and is generally referred to as the basecoat, anda clearcoat.

The basic compositions and functions of these layers and of the coatingcompositions needed to form these layers, i.e. electrocoat materials,so-called primer-surfacers, coating compositions which comprise colorpigments and/or effect pigments and are known as basecoat materials, andclearcoat materials, are known. For example, the electrocoat applied byelectrophoresis serves basically to protect the substrate fromcorrosion. The so-called primer-surfacer coat serves principally forprotection from mechanical stress, for example stone-chipping, andadditionally to level out unevenness in the substrate. The next coat,referred to as the basecoat, is principally responsible for the creationof esthetic properties such as color and/or effects such as flop, whilethe clearcoat which then follows serves particularly to impart scratchresistance and the gloss of the multicoat paint system.

These multicoat paint systems are generally produced by first applyingor depositing an electrocoat, especially a cathodic electrocoat, byelectrophoresis on the metallic substrate, for example an automobilebody. Prior to the deposition of the electrocoat, the metallic substratecan be pretreated in different ways; for example, it is possible toapply known conversion coatings such as phosphate coats, especially zincphosphate coats. The deposition process of electrocoating generallytakes place in appropriate electrocoating baths. After the application,the coated substrate is removed from the bath, optionally rinsed andflashed off and/or intermediately dried, and the electrocoat applied isfinally cured. The target film thicknesses are about 15 to 25micrometres. Subsequently, the so-called primer-surfacer is applieddirectly to the cured electrocoat, optionally flashed off and/orintermediately dried, and then cured. In order that the curedprimer-surfacer coat can fulfill the abovementioned tasks, target filmthicknesses are, for example, 25 to 45 micrometres. Subsequently, aso-called basecoat which comprises color pigments and/or effect pigmentsis applied directly to the cured primer-surfacer coat, and is optionallyflashed off and/or intermediately dried, and a clearcoat is applieddirectly to the basecoat thus produced without separate curing.Subsequently, the basecoat, and the clearcoat which has optionallylikewise been flashed off and/or intermediately dried beforehand, arejointly cured (wet-on-wet method). While the cured basecoat in principlehas comparatively low film thicknesses of, for example, 10 to 30micrometres, target film thicknesses for the cured clearcoat are, forexample, 30 to 60 micrometres, in order to achieve the performanceproperties described. Primer-surfacer, basecoat and clearcoat can beapplied, for example, via the application methods, which are known tothose skilled in the art, of pneumatic and/or electrostatic sprayapplication. Nowadays, primer-surfacer and basecoat are increasinglybeing used in the form of aqueous coating materials, for environmentalreasons at least.

Even though the multicoat paint systems thus produced can generally meetthe demands made by the automobile industry on performance propertiesand esthetic profile, the simplification of the comparatively complexproduction process described, for environmental and economic reasons, isnow the subject of increasing attention from the automobilemanufacturers.

For instance, there are approaches in which an attempt is made todispense with the separate curing step for the coating compositionapplied directly to the cured electrocoat (for the coating compositionreferred to as primer-surfacer in the context of the above-describedstandard method), and also at the same time to lower the film thicknessof the coating film produced from this coating composition. In thespecialist field, this coating film which is thus not cured separatelyis then frequently referred to as the basecoat (and no longer as theprimer-surfacer coat), or as the first basecoat as opposed to a secondbasecoat which is applied thereto. There are even some attempts tocompletely dispense with this coating film (in which case only aso-called basecoat is produced directly on the electrocoat, which isovercoated with a clearcoat without a separate curing step, meaning thata separate curing step is ultimately likewise dispensed with). Insteadof the separate curing step and an additional final curing step, thereis thus to be only a final curing step after application of all thecoating films applied to the electrocoat.

Specifically the omission of a separate curing step for the coatingcomposition applied directly to the electrocoat is very advantageousfrom an environmental and economic point of view. This is because itleads to an energy saving, and the overall production process can ofcourse run much more stringently and rapidly.

Instead of the separate curing step, it is thus advantageous that thecoating film produced directly on the electrocoat is flashed off only atroom temperature and/or intermediately dried at elevated temperatures,without conducting a curing operation, which is known to regularlyrequire elevated curing temperatures and/or long curing times.

It is problematic, however, that the required performance propertiesoften cannot be obtained nowadays in this form of production. A problemwhich often occurs in this context is that impact resistance, which isvery important specifically in automobiles, is not achieved to asatisfactory level. Impact resistance refers to the mechanicalresistance of coatings to rapid deformation. Of particularly highrelevance in this context is stone-chip resistance, meaning theresistance of a paint system to stones which hit the surface of thepaint system at high speed. This is because automotive paint systems areexposed particularly to this stone-chipping to a very intense degree.

It would accordingly be advantageous to have a method for producingmulticoat paint systems in which it is possible to dispense with aseparate curing step, as described above, for the coating compositionapplied directly to the electrocoat, and the multicoat paint systemproduced nevertheless has excellent impact resistance, especiallystone-chip resistance.

Problem

The problem addressed by the present invention was accordingly that offinding a method for producing a multicoat paint system on metallicsubstrates, in which the coating composition applied directly to theelectrocoat is not cured separately, but in which this coatingcomposition is instead cured in a joint curing step with further coatingfilms applied thereafter. In spite of this method simplification, theresulting multicoat paint systems should have excellent impactresistance, especially stone-chip resistance, such that the multicoatpaint systems especially meet the performance demands from theautomobile manufacturers and their customers.

Technical Solution

It has been found that the problems mentioned are solved by a novelmethod for producing a multicoat paint system (M) on a metallicsubstrate (S), comprising

(1) producing a cured electrocoat (E.1) on the metallic substrate (S) byelectrophoretic application of an electrocoat (e.1) to the substrate (S)and subsequent curing of the electrocoat (e.1),(2) producing (2.1) a basecoat (B.2.1) or (2.2) a plurality of directlysuccessive basecoats (B.2.2.x) directly on the cured electrocoat (E.1)by (2.1) applying an aqueous basecoat material (b.2.1) directly to theelectrocoat (E.1) or (2.2) applying a plurality of basecoat materials(b.2.2.x) in direct succession to the electrocoat (E.1),(3) producing a clearcoat (K) directly on (3.1) the basecoat (B.2.1) or(3.2) the uppermost basecoat (B.2.2.x) by applying a clearcoat material(k) directly to (3.1) the basecoat (B.2.1) or (3.2) the uppermostbasecoat (B.2.2.x),(4) jointly curing (4.1) the basecoat (B.2.1) and the clearcoat (K) or(4.2) the basecoats (B.2.2.x) and the clearcoat (K),whereinthe basecoat material (b.2.1) or at least one of the basecoat materials(b.2.2.x) comprises the following components:at least one aqueous dispersion comprising at least one copolymer (CP),said copolymer (CP) being preparable by

-   -   (i) initially charging an aqueous dispersion of at least one        polyurethane, and then    -   (ii) polymerizing a mixture of olefinically unsaturated monomers        in the presence of the polyurethane from (i),        -   in which        -   (a) a water-soluble initiator is used,        -   (b) the olefinically unsaturated monomers are metered in            such that a concentration of 6.0-% by weight, based on the            total amount of olefinically unsaturated monomers used for            polymerization, in the reaction solution is not exceeded            over the entire reaction time, and        -   (c) the mixture of the olefinically unsaturated monomers            comprises at least one polyolefinically unsaturated monomer,            at least one linear hydroxy-functional reaction product (R)            having an acid number less than 20 mg KOH/g, the preparation            of which involves using at least one compound (v) containing            two functional groups (v.a) and an aliphatic or araliphatic            hydrocarbyl radical (v.b) which is arranged between the            functional groups and has 12 to 70 carbon atoms,            and            at least one polyurethane resin (X), the preparation of            which involves using at least one compound (x.1) containing            at least one carboxylic acid group and at least one group            reactive toward isocyanate groups.

The abovementioned method is also referred to hereinafter as method ofthe invention, and accordingly forms part of the subject matter of thepresent invention. Preferred embodiments of the method of the inventioncan be found in the description which follows below and in the dependentclaims.

The present invention further provides a multicoat paint system whichhas been produced by the method of the invention.

The method of the invention allows the production of multicoat paintsystems without a separate curing step for the coating film produceddirectly on the electrocoat. For the sake of better clarity, thiscoating film is referred to as basecoat in the context of the presentinvention. Instead of separate curing, this basecoat is jointly curedtogether with any further basecoats beneath the clearcoat, and theclearcoat. In spite of this, the employment of the method according tothe invention results in multicoat paint systems having excellent impactresistance, especially stone-chip resistance. It is additionallypossible to form the corresponding basecoats with aqueous coatingcompositions, in order thus to satisfy environmental demands.

DETAILED DESCRIPTION

First of all, some of the terms used in the present invention will beelucidated.

The application of a coating composition to a substrate, or theproduction of a coating film on a substrate, are understood as follows.The respective coating composition is applied in such a way that thecoating film produced therefrom is arranged on the substrate, but neednot necessarily be in direct contact with the substrate. Other layers,for example, may also be arranged between the coating film and thesubstrate. For example, in stage (1), the cured electrocoat (E.1) isproduced on the metallic substrate (S), but a conversion coating asdescribed below, such as a zinc phosphate coating, may also be arrangedbetween the substrate and the electrocoat.

The same principle applies to the application of a coating composition(b) to a coating film (A) produced by means of another coatingcomposition (a), or to the production of a coating film (B) on anothercoating film (A) arranged, for example, on the metallic substrate (S).The coating film (B) need not necessarily be in contact with the coatinglayer (A), but merely has to be arranged above it, i.e. on the side ofthe coating film (A) facing away from the metallic substrate.

In contrast, the application of a coating composition directly to asubstrate, or the production of a coating film directly on a substrate,is understood as follows. The respective coating composition is appliedin such a way that the coating film produced therefrom is arranged onthe substrate and is in direct contact with the substrate. Thus, moreparticularly, no other layer is arranged between coating film andsubstrate. Of course, the same applies to the application of a coatingcomposition (b) directly to a coating film (A) produced by means ofanother coating composition (a), or to the production of a coating film(B) directly on another coating film (A) arranged, for example, on themetallic substrate (S). In this case, the two coating films are indirect contact, i.e. are arranged directly one on top of the other. Moreparticularly, there is no further layer between the coating films (A)and (B).

Of course, the same principle applies to directly successive applicationof coating compositions, or the production of directly successivecoating films.

In the context of the present invention, “flashing off”, “intermediatelydrying” and “curing” are understood to have the meanings familiar to theperson skilled in the art in connection with methods for production ofmulticoat paint systems.

Thus, the term “flashing off” is understood in principle as a term forthe vaporization, or permitting vaporization, of organic solvents and/orwater in a coating composition applied in the production of a paintsystem, usually at ambient temperature (i.e. room temperature), forexample 15 to 35° C. for a period of, for example, 0.5 to 30 min. Duringthe flash-off operation, organic solvents and/or water present in thecoating composition applied thus vaporize. Since the coating compositionis still free-flowing at least directly after the application and oncommencement of the flash-off operation, it can run during the flash-offoperation. This is because at least a coating composition applied byspray application is generally applied in droplet form and not inhomogeneous thickness. However, it is free-flowing by virtue of theorganic solvents and/or water present and can thus form a homogeneous,smooth coating film by running. At the same time, organic solventsand/or water vaporize gradually, such that a comparatively smoothcoating film has formed after the flash-off phase, containing less waterand/or solvent compared to the coating composition applied. After theflash-off operation, the coating film, however, is still not in a stateready for use. For example, it is no longer free-flowing, but is stillsoft and/or tacky, and in some cases only partly dried. Moreparticularly, the coating film still has not cured as described below.

Intermediate drying is thus likewise understood to mean vaporization, orpermitting vaporization, of organic solvents and/or water in a coatingcomposition applied in the production of a paint system, usually at atemperature elevated relative to ambient temperature, for example of 40to 90° C., for a period of, for example, 1 to 60 min. In theintermediate drying operation too, the coating composition applied willthus lose a proportion of organic solvents and/or water. With regard toa particular coating composition, it is generally the case that theintermediate drying, compared to the flash-off, takes place at, forexample, higher temperatures and/or for a longer period, such that, incomparison to the flash-off, a higher proportion of organic solventsand/or water escapes from the coating film applied. However, theintermediate drying does not give a coating film in a state ready foruse either, i.e. a cured coating film as described below. A typicalsequence of flash-off and intermediate drying operations would involve,for example, flashing off a coating film applied at ambient temperaturefor 5 min and then intermediately drying it at 80° C. for 10 min.However, no conclusive delimitation of the two terms is either necessaryor intended. Purely for the sake of clarity, these terms are used tomake it clear that a curing operation described below may be preceded byvariable and sequential conditioning of a coating film inwhich—depending on the coating composition, the vaporization temperatureand vaporization time—a higher or lower proportion of the organicsolvents and/or water present in the coating composition can vaporize.As the case may be, a proportion of the polymers present in the coatingcompositions as binders, even at this early stage, can crosslink orinterloop as described below. However, neither the flash-off nor theintermediate drying operation gives a ready-to-use coating film, as isaccomplished by curing described below. Accordingly, curing is clearlydelimited from the flash-off and intermediate drying operations.

Accordingly, curing of a coating film is understood to mean theconversion of such a film to the ready-to-use state, i.e. to a state inwhich the substrate provided with the respective coating film can betransported, stored and used as intended. More particularly, a curedcoating film is no longer soft or tacky, but has been conditioned as asolid coating film which does not undergo any further significant changein its properties, such as hardness or adhesion on the substrate, evenunder further exposure to curing conditions as described below.

As is well known, coating compositions can in principle be curedphysically and/or chemically, according to the components present, suchas binders and crosslinking agents. In the case of chemical curing,thermochemical curing and actinochemical curing are options. If it isthermochemically curable, a coating composition may be self-crosslinkingand/or externally crosslinking. The statement that a coating compositionis self-crosslinking and/or externally crosslinking in the context ofthe present invention should be understood to mean that this coatingcomposition comprises polymers as binders and optionally crosslinkingagents, which can correspondingly crosslink with one another. Theunderlying mechanisms and usable binders and crosslinking agents aredescribed below.

In the context of the present invention, “physically curable” or theterm “physical curing” means the formation of a cured coating filmthrough release of solvent from polymer solutions or polymerdispersions, the curing being achieved through interlooping of polymerchains.

In the context of the present invention, “thermochemically curable” orthe term “thermochemical curing” means the crosslinking, initiated bychemical reaction of reactive functional groups, of a paint film(formation of a cured coating film), it being possible to provide theactivation energy for these chemical reactions through thermal energy.This can involve reaction of different, mutually complementaryfunctional groups with one another (complementary functional groups)and/or formation of the cured layer based on the reaction ofautoreactive groups, i.e. functional groups which inter-react withgroups of the same kind. Examples of suitable complementary reactivefunctional groups and autoreactive functional groups are known, forexample, from German patent application DE 199 30 665 A1, page 7 line 28to page 9 line 24.

This crosslinking may be self-crosslinking and/or external crosslinking.If, for example, the complementary reactive functional groups arealready present in an organic polymer used as a binder, for example apolyester, a polyurethane or a poly(meth)acrylate, self-crosslinking ispresent. External crosslinking is present, for example, when a (first)organic polymer containing particular functional groups, for examplehydroxyl groups, reacts with a crosslinking agent known per se, forexample a polyisocyanate and/or a melamine resin. The crosslinking agentthus contains reactive functional groups complementary to the reactivefunctional groups present in the (first) organic polymer used as thebinder.

Especially in the case of external crosslinking, the one-component andmulticomponent systems, especially two-component systems, known per seare useful.

In one-component systems, the components to be crosslinked, for exampleorganic polymers as binders and crosslinking agents, are presentalongside one another, i.e. in one component. A prerequisite for this isthat the components to be crosslinked react with one another, i.e. enterinto curing reactions, only at relatively high temperatures of, forexample, above 100° C. Otherwise, the components to be crosslinked wouldhave to be stored separately from one another and only be mixed with oneanother shortly before application to a substrate, in order to avoidpremature, at least partial thermochemical curing (cf. two-componentsystems). An example of a combination is that of hydroxy-functionalpolyesters and/or polyurethanes with melamine resins and/or blockedpolyisocyanates as crosslinking agents.

In two-component systems, the components to be crosslinked, for examplethe organic polymers as binders and the crosslinking agents, are presentseparately in at least two components which are combined only shortlyprior to application. This form is chosen when the components to becrosslinked react with one another even at ambient temperatures orslightly elevated temperatures of, for example, 40 to 90° C. An exampleof a combination is that of hydroxy-functional polyesters and/orpolyurethanes and/or poly(meth)acrylates with free polyisocyanates ascrosslinking agents.

It is also possible that an organic polymer as binder has bothself-crosslinking and externally crosslinking functional groups, and isthen combined with crosslinking agents.

In the context of the present invention, “actinochemically curable” orthe term “actinochemical curing” is understood to mean the fact thatcuring is possible using actinic radiation, namely electromagneticradiation such as near infrared (NIR) and UV radiation, especially UVradiation, and corpuscular radiation such as electron beams for curing.Curing by UV radiation is commonly initiated by radical or cationicphotoinitiators. Typical actinically curable functional groups arecarbon-carbon double bonds, for which generally free-radicalphotoinitiators are used. Actinic curing is thus likewise based onchemical crosslinking.

Of course, in the curing of a coating composition described aschemically curable, it is always also possible for physical curing tooccur, i.e. interlooping of polymer chains. Nevertheless, such a coatingcomposition is described as chemically curable in that case.

It follows from the above that, according to the nature of the coatingcomposition and the components present therein, curing is brought aboutby different mechanisms which, of course, also necessitate differentconditions in the curing, more particularly different curingtemperatures and curing times.

In the case of a purely physically curing coating composition, curing iseffected preferably between 15 and 90° C. over a period of 2 to 48hours. In this case, curing may thus differ from the flash-off and/orintermediate drying operation merely by the duration of the conditioningof the coating film. Moreover, differentiation between flashing-off andintermediate drying is not meaningful. It would be possible, forexample, first to flash off or intermediately dry a coating filmproduced by applying a physically curable coating composition at 15 to35° C. for a period of, for example, 0.5 to 30 min, and then to keep itat 50° C. for a period of 5 hours.

Preferably, the coating compositions for use in the method of theinvention, i.e. electrocoat materials, aqueous basecoat materials andclearcoat materials, however, are at least thermochemically curable,especially preferably thermochemically curable and externallycrosslinking.

In principle, and within the context of the present invention, thecuring of one-component systems is performed preferably at temperaturesof 100 to 250° C., preferably 100 to 180° C., for a period of 5 to 60min, preferably 10 to 45 min, since these conditions are generallynecessary to convert the coating film to a cured coating film throughchemical crosslinking reactions. Accordingly, any flash-off and/orintermediate drying phase which precedes the curing is effected at lowertemperatures and/or for shorter periods. In such a case, for example,flashing-off can be effected at 15 to 35° C. for a period of, forexample, 0.5 to 30 min, and/or intermediate drying at a temperature of,for example, 40 to 90° C. for a period of, for example, 1 to 60 min.

In principle, and within the context of the present invention, thecuring of two-component systems is performed at temperatures of, forexample, 15 to 90° C., preferably 40 to 90° C., for a period of 5 to 80min, preferably 10 to 50 min. Accordingly, any flash-off and/orintermediate drying phase which precedes the curing is effected at lowertemperatures and/or for shorter periods. In such a case, for example, itis no longer meaningful to distinguish between the terms “flash-off” and“intermediate drying”. Any flash-off and/or intermediate drying phasewhich precedes the curing may proceed, for example, at 15 to 35° C. fora period of, for example, 0.5 to 30 min, but at least at lowertemperatures and/or for shorter periods than the curing which thenfollows.

This of course does not rule out curing of a two-component system athigher temperatures. For example, in step (4) of the method of theinvention, which is described in detail below, a basecoat or a pluralityof basecoats is/are cured together with a clearcoat. If bothone-component and two-component systems are present within the films,for example a one-component basecoat and a two-component clearcoat, thejoint curing is of course guided by the curing conditions needed for theone-component system.

All the temperatures exemplified in the context of the present inventionare understood as the temperature of the room in which the coatedsubstrate is present. What is thus not meant is that the substrateitself must have the particular temperature.

If reference is made in the context of the present invention to anofficial standard without reference to the official period of validity,this of course means the version of the standard current at the filingdate or, if no current version exists at this date, the last currentversion.

The Method of the Invention

In the method of the invention, a multicoat paint system is formed on ametallic substrate (S).

Useful metallic substrates (S) include, in principle, substratescomprising or consisting of, for example, iron, aluminum, copper, zinc,magnesium and alloys thereof, and steel in a wide variety of differentforms and compositions. Preference is given to iron and steelsubstrates, for example typical iron and steel substrates as used in theautomobile industry. The substrates may in principle be in any form,meaning that they may, for example, be simple sheets or else complexcomponents, such as, more particularly, automobile bodies and partsthereof.

Prior to stage (1) of the method of the invention, the metallicsubstrates (S) can be pretreated in a manner known per se, i.e., forexample, cleaned and/or provided with known conversion coatings.Cleaning can be effected mechanically, for example by means of wiping,grinding and/or polishing, and/or chemically by means of etching methodsby surface etching in acid or alkali baths, for example by means ofhydrochloric acid or sulfuric acid. Of course, cleaning with organicsolvents or aqueous detergents is also possible. Pretreatment byapplication of conversion coatings, especially by means of phosphationand/or chromation, preferably phosphation, may likewise take place.Preferably, the metallic substrates are at least conversion-coated,especially phosphated, preferably by a zinc phosphation.

In stage (1) of the method of the invention, a cured electrocoat (E.1)is produced on the metallic substrate (S) by electrophoretic applicationof an electrocoat material (e.1) to the substrate (S) and subsequentcuring of the electrocoat material (e.1).

The electrocoat material (e.1) used in stage (1) of the method of theinvention may be a cathodic or anodic electrocoat material. It ispreferably a cathodic electrocoat material. Electrocoat materials havelong been known to those skilled in the art. These are aqueous coatingmaterials comprising anionic or cationic polymers as binders. Thesepolymers contain functional groups which are potentially anionic, i.e.can be converted to anionic groups, for example carboxylic acid groups,or functional groups which are potentially cationic, i.e. can beconverted to cationic groups, for example amino groups. The conversionto charged groups is generally achieved through the use of appropriateneutralizing agents (organic amines (anionic), organic carboxylic acidssuch as formic acid (cationic)), which then gives rise to the anionic orcationic polymers. The electrocoat materials generally, and thuspreferably additionally, comprise typical anticorrosion pigments. Thecathodic electrocoat materials preferred in the context of the inventioncomprise preferably cationic polymers as binders, especiallyhydroxy-functional polyether amines, which preferably have aromaticstructural units. Such polymers are generally obtained by reaction ofappropriate bisphenol-based epoxy resins with amines, for example mono-and dialkylamines, alkanolamines and/or dialkylaminoalkylamines. Thesepolymers are especially used in combination with blocked polyisocyanatesknown per se. Reference is made by way of example to the electrocoatmaterials described in WO 9833835 A1, WO 9316139 A1, WO 0102498 A1 andWO 2004018580 A1.

The electrocoat material (e.1) is thus preferably an at leastthermochemically curable coating material, and is especially externallycrosslinking. The electrocoat material (e.1) is preferably aone-component coating composition. Preferably, the electrocoat material(e.1) comprises a hydroxy-functional epoxy resin as a binder and a fullyblocked polyisocyanate as a crosslinking agent. The epoxy resin ispreferably cathodic, and especially contains amino groups.

The electrophoretic application of such an electrocoat material (e.1)which takes place in stage (1) of the method of the invention is alsoknown. The application proceeds by electrophoresis. This means thatmetallic workpiece to be coated is first dipped into a dip bathcontaining the coating material, and an electrical DC field is appliedbetween the metallic workpiece and a counterelectrode. The workpiecethus functions as an electrode; the nonvolatile constituents of theelectrocoat material migrate, because of the described charge of thepolymers used as binders, through the electrical field to the substrateand are deposited on the substrate, forming a electrocoat film. Forexample, in the case of a cathodic electrocoat, the substrate is thusconnected as the cathode, and the hydroxide ions which form therethrough water electrolysis neutralize the cationic binder, such that itis deposited on the substrate and forms an electrocoat layer. In thatcase, application is thus accomplished through the electrophoreticdipping method.

After the electrolytic application of the electrocoat material (e.1),the coated substrate (S) is removed from the bath, optionally rinsed offwith, for example, water-based rinse solutions, then optionally flashedoff and/or intermediately dried, and the electrocoat material applied isfinally cured.

The electrocoat material (e.1) applied (or the as yet uncuredelectrocoat applied) is flashed off, for example, at 15 to 35° C. for aperiod of, for example, 0.5 to 30 min and/or intermediately dried at atemperature of preferably 40 to 90° C. for a period of, for example, 1to 60 min.

The electrocoat material (e.1) applied to the substrate (or the as yetuncured electrocoat applied) is preferably cured at temperatures of 100to 250° C., preferably 140 to 220° C., for a period of 5 to 60 min,preferably 10 to 45 min, which produces the cured electrocoat (E.1).

The flash-off, intermediate drying and curing conditions specified applyespecially to the preferred case that the electrocoat material (e.1) isa one-component coating composition thermochemically curable asdescribed above. However, this does not rule out the possibility thatthe electrocoat material is a coating composition curable in another wayand/or that other flash-off, intermediate drying and curing conditionsare used.

The layer thickness of the cured electrocoat is, for example, to 40micrometres, preferably 15 to 25 micrometres. All the film thicknessesstated in the context of the present invention should be understood asdry film thicknesses. The film thickness is thus that of the cured filmin question. Thus, if it is stated that a coating material is applied ina particular film thickness, this should be understood to mean that thecoating material is applied such that the stated film thickness resultsafter the curing.

In stage (2) of the method of the invention, (2.1) a basecoat (B.2.1) isproduced or (2.2) a plurality of directly successive basecoats (B.2.2.x)are produced. The coats are produced by applying (2.1) an aqueousbasecoat material (b.2.1) directly to the cured electrocoat (E.1) or(2.2) directly successively applying a plurality of basecoat materials(b.2.2.x) to the cured electrocoat (E.1).

The directly successive application of a plurality of basecoat materials(b.2.2.x) to the cured electrocoat (E.1) is thus understood to mean thata first basecoat material is first applied directly to the electrocoatand then a second basecoat material is applied directly to the coat ofthe first basecoat material. Any third basecoat material is then applieddirectly to the coat of the second basecoat material. This operation canthen be repeated analogously for further basecoat materials (i.e. afourth, fifth, etc. basecoat).

The basecoat (B.2.1) or the first basecoat (B.2.2.x), after theproduction, is thus arranged directly on the cured electrocoat (E.1).

The terms “basecoat material” and “basecoat” in relation to the coatingcompositions applied and coating films produced in stage (2) of themethod of the invention are used for the sake of better clarity. Thebasecoats (B.2.1) and (B.2.2.x) are not cured separately, but rather arecured together with the clearcoat material. The curing is thus effectedanalogously to the curing of so-called basecoat materials used in thestandard method described by way of introduction. More particularly, thecoating compositions used in stage (2) of the method of the inventionare not cured separately, like the coating compositions referred to asprimer-surfacers in the context of the standard method.

The aqueous basecoat material (b.2.1) used in stage (2.1) is describedin detail below. However, it is preferably at least thermochemicallycurable, and it is especially externally crosslinking. Preferably, thebasecoat material (b.2.1) is a one-component coating composition.Preferably, the basecoat material (b.2.1) comprises a combination of atleast one hydroxy-functional polymer as a binder, selected from thegroup consisting of polyurethanes, polyesters, polyacrylates andcopolymers of the polymers mentioned, for examplepolyurethane-polyacrylates, and at least one melamine resin as acrosslinking agent.

The basecoat material (b.2.1) can be applied by methods known to thoseskilled in the art for application of liquid coating compositions, forexample by dipping, bar coating, spraying, rolling or the like.Preference is given to employing spray application methods, for examplecompressed air spraying (pneumatic application), airless spraying,high-speed rotation, electrostatic spray application (ESTA), optionallyin association with hot-spray application, for example hot-air spraying.Most preferably, the basecoat material (b.2.1) is applied by means ofpneumatic spray application or electrostatic spray application. Theapplication of the basecoat material (b.2.1) thus produces a basecoat(B.2.1), i.e. a coat of the basecoat material (b.2.1) applied directlyto the electrocoat (E.1).

After application, the basecoat material (b.2.1) applied, or thecorresponding basecoat (B2.1) is flashed off, for example, at 15 to 35°C. for a period of, for example, 0.5 to min and/or intermediately driedat a temperature of preferably 40 to 90° C. for a period of, forexample, 1 to 60 min. Preference is given to first flashing off at 15 to35° C. for a period of 0.5 to 30 min and then intermediately drying at40 to 90° C. for a period of, for example, 1 to 60 min. The flash-offand intermediate drying conditions described apply especially to thepreferred case that the basecoat material (b.2.1) is a thermochemicallycurable one-component coating composition. However, this does not ruleout the possibility that the basecoat material (b.2.1) is a coatingcomposition curable in another way and/or that other flash-off and/orintermediate drying conditions are used.

The basecoat (B.2.1) is not cured within stage (2) of the method of theinvention, i.e. is preferably not exposed to temperatures of more than100° C. for a period of longer than 1 min, and especially preferably isnot exposed to temperatures of more than 100° C. at all. This is clearlyand unambiguously apparent from stage (4) of the method of theinvention, described below. Since the basecoat is not cured until stage(4), it cannot be cured at the earlier stage (2), since curing in stage(4) would not be possible in that case.

The aqueous basecoat materials (b.2.2.x) used in stage (2.2) of themethod of the invention are also described in detail below. At least oneof the basecoat materials (b.2.2.x) used in stage (2.2), preferably allof those used in stage (2.2), however, are preferably at leastthermochemically curable, especially preferably externally crosslinking.Preferably, at least one basecoat material (b.2.2.x) is a one-componentcoating composition; this preferably applies to all the basecoatmaterials (b.2.2.x). Preferably, at least one of the basecoat materials(b.2.2.x) comprises a combination of at least one hydroxy-functionalpolymer as a binder, selected from the group consisting ofpolyurethanes, polyesters, polyacrylates and copolymers of the polymersmentioned, for example polyurethane-polyacrylates, and at least onemelamine resin as a crosslinking agent. This preferably applies to allthe basecoat materials (b.2.2.x).

The basecoat materials (b.2.2.x) can be applied by methods known tothose skilled in the art for application of liquid coating compositions,for example by dipping, bar coating, spraying, rolling or the like.Preference is given to employing spray application methods, for examplecompressed air spraying (pneumatic application), airless spraying,high-speed rotation, electrostatic spray application (ESTA), optionallyin association with hot-spray application, for example hot-air (hotspraying). Most preferably, the basecoat materials (b.2.2.x) are appliedby means of pneumatic spray application and/or electrostatic sprayapplication.

In stage (2.2) of the method of the invention, the naming system whichfollows is suggested. The basecoat materials and basecoats are generallydesignated by (b.2.2.x) and (B.2.2.x), while the x can be replaced byother appropriate letters in the naming of the specific individualbasecoat materials and basecoats.

The first basecoat material and the first basecoat can be designated bya, and the uppermost basecoat material and the uppermost basecoat can bedesignated by z. These two basecoat materials or basecoats are alwayspresent in stage (2.2). Any coats arranged in between can be designatedserially with b, c, d and so forth.

The application of the first basecoat material (b.2.2.a) thus produces abasecoat (B.2.2.a) directly on the cured electrocoat (E.1). The at leastone further basecoat (B.2.2.x) is then produced directly on the basecoat(B.2.2.a). If a plurality of further basecoats (B.2.2.x) are produced,these are produced in direct succession. For example, it is possible forexactly one further basecoat (B.2.2.x) to be produced, in which casethis is then arranged directly below the clearcoat (K) in the multicoatpaint system ultimately produced, and can thus be referred to as thebasecoat (B.2.2.z) (cf. also FIG. 2). It is also possible, for example,that two further basecoats (B.2.2.x) are produced, in which case thecoat produced directly on the basecoat (B.2.2.a) can be designated as(B.2.2.b), and the coat finally arranged directly below the clearcoat(K) in turn as (B.2.2.z) (cf. also FIG. 3).

The basecoat materials (b.2.2.x) may be identical or different. It isalso possible to produce a plurality of basecoats (B.2.2.x) with thesame basecoat material, and one or more further basecoats (B.2.2.x) withone or more other basecoat materials.

The basecoat materials (b.2.2.x) applied are generally flashed offand/or intermediately dried separately and/or together. In stage (2.2)too, preference is given to flashing off at 15 to 35° C. for a period of0.5 to 30 min and intermediately drying at 40 to 90° C. for a period of,for example, 1 to 60 min. The sequence of flash-off and/or intermediatedrying operations on individual or plural basecoats (B.2.2.x) can beadjusted according to the demands of the individual case. Theabove-described preferred flash-off and intermediate drying conditionsapply especially to the preferred case that at least one basecoatmaterial (b.2.2.x), preferably all the basecoat materials (b.2.2.x),comprise(s) thermochemically curable one-component coating compositions.However, this does not rule out the possibility that the basecoatmaterials (b.2.2.x) are coating compositions curable in another wayand/or that other flash-off and/or intermediate drying conditions areused.

Some preferred variants of the basecoat sequences of the basecoatmaterials (b.2.2.x) are elucidated as follows.

Variant a) It is possible to produce a first a basecoat by electrostaticspray application (ESTA) of a first basecoat material, and to produce afurther basecoat directly on the first basecoat by pneumatic sprayapplication of the same basecoat material. Although the two basecoatsare thus based on the same basecoat material, the application isobviously effected in two stages, such that the basecoat material inquestion in the method of the invention corresponds to a first basecoatmaterial (b.2.2.a) and a further basecoat material (b.2.2.z). Before thepneumatic application, the first basecoat is preferably flashed offbriefly, for example at 15 to 35° C. for 0.5 to 3 min. After thepneumatic application, flash-off is then effected at, for example, 15 to35° C. for 0.5 to 30 min, and then intermediate drying at 40 to 90° C.for a period of 1 to 60 min. The structure described is frequently alsoreferred to as a one-coat basecoat structure produced in twoapplications (once by ESTA, once pneumatically). Since, however,especially in real OEM finishing, the technical circumstances in apainting facility mean that a certain timespan always passes between thefirst application and the second application, in which the substrate,for example the automobile body, is conditioned at to 35° C., forexample, and hence is flashed off, the characterization of thisstructure as a two-coat basecoat structure is clearer in a formal sense.This variant of stage (2.2) is preferably chosen when the basecoatmaterial (b.2.2.x) used (or the two identical basecoat materials(b.2.2.a) and (b.2.2.z) used) comprises effect pigments as describedbelow. While ESTA application can guarantee good material transfer oronly a small paint loss in the application, the pneumatic applicationwhich then follows achieves good alignment of the effect pigments andhence good properties of the overall paint system, especially a highflop.

Variant b) It is also possible to produce a first basecoat byelectrostatic spray application (ESTA) of a first basecoat materialdirectly on the cured electrocoat, to flash off and/or intermediatelydry said first basecoat material, and then to produce a second basecoatby direct application of a second basecoat material other than the firstbasecoat material. In this case, the second basecoat material can also,as described in variant a), be applied first by electrostatic sprayapplication (ESTA) and then by pneumatic spray application, as a resultof which two directly successive basecoats, both based on the secondbasecoat material, are produced directly on the first basecoat. Betweenand/or after the applications, flashing-off and/or intermediate dryingis of course again possible. Variant (b) of stage (2.2) is preferablyselected when a color-preparing basecoat as described below is first tobe produced directly on the electrocoat and then, in turn, a doubleapplication of a basecoat material comprising effect pigments or anapplication of a basecoat material comprising chromatic pigments is tobe effected. In that case, the first basecoat is based on thecolor-preparing basecoat material, the second and third basecoats on thebasecoat material comprising effect pigments, or the one furtherbasecoat on a further basecoat material comprising chromatic pigments.

Variant c) It is likewise possible to produce three basecoats directlyin succession directly on the cured electrocoat, in which case thebasecoats are based on three different basecoat materials. For example,it is possible to produce a color-preparing basecoat, a further coatbased on a basecoat material comprising color pigments and/or effectpigments, and a further coat based on a second basecoat materialcomprising color pigments and/or effect pigments. Between and/or afterthe individual applications, and/or after all three applications, it isagain possible to flash off and/or intermediately dry.

Embodiments preferred in the context of the present invention thusinclude production of two or three basecoats in stage (2.2) of themethod of the invention, and preference is given in this context toproduction of two directly successive basecoats using the same basecoatmaterial, and very particular preference to production of the first ofthese two basecoats by ESTA application and the second of these twobasecoats by pneumatic application. In that case, it is preferable inthe case of production of a three-coat basecoat structure that thebasecoat produced directly on the cured electrocoat is based on acolor-preparing basecoat material. The second and third coats are basedeither on one and the same basecoat material, which preferably compriseseffect pigments, or on a first basecoat material comprising colorpigments and/or effect pigments and a different second basecoat materialcomprising color pigments and/or effect pigments.

The basecoats (B.2.2.x) are not cured within stage (2) of the method ofthe invention, i.e. are preferably not exposed to temperatures of morethan 100° C. for a period of longer than 1 min, and preferably are notexposed to temperatures of more than 100° C. at all. This is clearly andunambiguously apparent from stage (4) of the method of the invention,described below. Since the basecoats are not cured until stage (4), theycannot be cured at the earlier stage (2), since curing in stage (4)would not be possible in that case.

The application of the basecoat materials (b.2.1) and (b.2.2.x) iseffected in such a way that the basecoat (B.2.1) and the individualbasecoats (B.2.2.x), after the curing effected in stage (4), have a coatthickness of, for example, 5 to 40 micrometres, preferably 6 to 35micrometres, especially preferably 7 to 30 micrometres. In stage (2.1),preferably higher coat thicknesses of 15 to 40 micrometres, preferably20 to 35 micrometres, are produced. In stage (2.2), the individualbasecoats have, if anything, comparatively lower coat thicknesses, inwhich case the overall structure again has coat thicknesses within theorder of magnitude of the one basecoat (B.2.1). For example, in the caseof two basecoats, the first basecoat (B.2.2.a) preferably has coatthicknesses of 5 to 35 and especially 10 to 30 micrometres, and thesecond basecoat (B.2.2.z) preferably has coat thicknesses of 5 to 30micrometres, especially 10 to 25 micrometres.

In stage (3) of the method of the invention, a clearcoat (K) is applieddirectly to (3.1) the basecoat (B.2.1) or (3.2) the uppermost basecoat(B.2.2.z). This production is effected by appropriate application of aclearcoat material (k).

The clearcoat material (k) may in principle be any transparent coatingcomposition known to the person skilled in the art in this context. Thisincludes aqueous or solventborne transparent coating compositions, whichmay be formulated either as one-component or two-component coatingcompositions, or multicomponent coating compositions. In addition,powder slurry clearcoat materials are also suitable. Preference is givento solvent-based clearcoat materials.

The clearcoat materials (k) used may especially be thermochemicallyand/or actinochemically curable. More particularly, they arethermochemically curable and externally crosslinking. Preference isgiven to two-component clearcoat materials.

The transparent coating compositions thus typically and preferablycomprise at least one (first) polymer as a binder having functionalgroups, and at least one crosslinker having a functionalitycomplementary to the functional groups of the binder. Preference isgiven to using at least one hydroxy-functional poly(meth)acrylatepolymer as a binder and a polyisocyanate as a crosslinking agent.

Suitable clearcoat materials are described, for example, in WO2006042585 A1, WO 2009077182 A1 or else WO 2008074490 A1.

The clearcoat material (k) is applied by methods known to those skilledin the art for application of liquid coating compositions, for exampleby dipping, bar coating, spraying, rolling or the like. Preference isgiven to employing spray application methods, for example compressed airspraying (pneumatic application), and electrostatic spray application(ESTA).

After application, the clearcoat material (k) or the correspondingclearcoat (K) is flashed off or intermediately dried at 15 to 35° C. fora period of 0.5 to 30 min. Flash-off and intermediate drying conditionsof this kind apply especially to the preferred case that the clearcoatmaterial (k) is a thermochemically curable two-component coatingcomposition. However, this does not rule out the possibility that theclearcoat material (k) is a coating composition curable in another wayand/or that other flash-off and/or intermediate drying conditions areused.

The application of the clearcoat material (k) is effected in such a waythat the clearcoat, after the curing effected in stage (4), has a coatthickness of, for example, 15 to 80 micrometres, preferably 20 to 65micrometres, especially preferably 25 to 60 micrometres.

In stage (4) of the method of the invention, there is joint curing of(4.1) the basecoat (B.2.1) and the clearcoat (K) or (4.2) the basecoats(B.2.2.x) and the clearcoat (K).

The joint curing is preferably effected at temperatures of 100 to 250°C., preferably 100 to 180° C., for a period of 5 to 60 min, preferably10 to 45 min. Curing conditions of this kind apply especially to thepreferred case that the basecoat (3.2.1) or at least one of thebasecoats (B.2.2.x), preferably all the basecoats (B.2.2.x), is/arebased on a thermochemically curable one-component coating composition.This is because, as described above, such conditions are generallyrequired to achieve curing as described above in such a one-componentcoating composition. If the clearcoat material (k) is, for example,likewise a thermochemically curable one-component coating composition,the clearcoat (K) in question is of course likewise cured under theseconditions. The same obviously applies to the preferred case that theclearcoat material (k) is a thermochemically curable two-componentcoating composition.

However, the above statements do not rule out the possibility that thebasecoat materials (b.2.1) and (b.2.2.x) and the clearcoat materials (k)are coating compositions curable in another way and/or that other curingconditions are used.

After stage (4) of the method of the invention has ended, the result isa multicoat paint system of the invention.

The Basecoat Materials for Use in Accordance with the Invention

The basecoat material (b.2.1) for use in accordance with the inventioncomprises a specific aqueous dispersion comprising at least one specificcopolymer (CP), preferably exactly one copolymer (CP).

A copolymer in the context of the present invention refers to polymersformed from different polymer types, for example a polyurethane and a(meth)acrylate polymer. This explicitly includes both polymerscovalently bonded to one another and those in which the various polymersare bonded to one another by adhesion. Combinations of both kinds ofbonding are also covered by this definition. The term “(meth)acrylate”covers acrylates, methacrylates and mixtures thereof.

The copolymer (CP) is preparable by

-   -   (i) initially charging an aqueous dispersion of at least one        polyurethane, and then    -   (ii) polymerizing a mixture of olefinically unsaturated monomers        in the presence of the polyurethane from (i), in which        -   a. a water-soluble initiator is used,        -   b. the olefinically unsaturated monomers are metered in such            that a concentration of 6.0% by weight, based on the total            amount of olefinically unsaturated monomers used for            polymerization, in the reaction solution is not exceeded            over the entire reaction time, and        -   c. the mixture of the olefinically unsaturated monomers            comprises at least one polyolefinically unsaturated monomer.

In the first preparation step, an aqueous dispersion of a polyurethaneresin is initially charged.

Suitable saturated or unsaturated polyurethane resins are described, forexample, in

-   -   German patent application DE 199 48 004 A1, page 4 line 19 to        page 11 line 29 (polyurethane prepolymer B1),    -   European patent application EP 0 228 003 A1, page 3 line 24 to        page 5 line 40,    -   European patent application EP 0 634 431 A1, page 3 line 38 to        page 8 line 9, or    -   international patent application WO 92/15405, page 2 line 35 to        page 10 line 32.

The polyurethane resin is prepared using firstly, preferably, thealiphatic, cycloaliphatic, aliphatic-cycloaliphatic, aromatic,aliphatic-aromatic and/or cycloaliphatic-aromatic polyisocyanates knownto those skilled in the art. Particular preference is given to aliphaticand aliphatic-cycloaliphatic polyurethane resins.

The alcohol components used for the preparation of the polyurethaneresins are preferably the saturated and unsaturated polyols known tothose skilled in the art, and optionally, in minor amounts, alsomonoalcohols. More particularly, diols and, optionally in minor amounts,triols are used to introduce branches. Examples of suitable polyols aresaturated or olefinically unsaturated polyester polyols and/or polyetherpolyols. More particularly, the polyols used are polyester polyols,especially those having a number-average molecular weight of 400 to 5000g/mol. Unless specifically indicated otherwise, the number-averagemolecular weight in the context of the present invention is determinedby means of vapor pressure osmosis. Measurement was effected using avapor pressure osmometer (model 10.00 from Knauer) on concentrationseries of the component under investigation in toluene at 50° C., withbenzophenone as calibration substance for determination of theexperimental calibration constant of the instrument employed (inaccordance with E. Schröder, G. Müller, K.-F. Arndt, “Leitfaden derPolymercharakterisierung”, Akademie-Verlag, Berlin, pp. 47-54, 1982, inwhich benzil was used as calibration substance).

The polyurethane initially charged in aqueous dispersion is preferably ahydrophilically stabilized polyurethane. For hydrophilic stabilizationand/or to increase dispersibility in aqueous medium, the polyurethaneresin preferably present may contain particular ionic groups and/orgroups which can be converted to ionic groups (potentially ionicgroups). Polyurethane resins of this kind are referred to in the contextof the present invention as ionically hydrophilically stabilizedpolyurethane resins. Likewise present may be nonionic hydrophilicallymodifying groups. Preferred, however, are the ionically hydrophilicallystabilized polyurethanes. In more precise terms, the modifying groupsare alternatively

-   -   functional groups which can be converted to cations by        neutralizing agents and/or quaternizing agents, and/or cationic        groups (cationic modification) or    -   functional groups which can be converted to anions by        neutralizing agents, and/or anionic groups (anionic        modification) or    -   nonionic hydrophilic groups (nonionic modification) or    -   combinations of the aforementioned groups.

As the skilled person is aware, the functional groups for cationicmodification are, for example, primary, secondary and/or tertiary aminogroups, secondary sulfide groups and/or tertiary phosphine groups, moreparticularly tertiary amino groups and secondary sulfide groups(functional groups which can be converted to cationic groups byneutralizing agents and/or quaternizing agents). Mention should also bemade of the cationic groups—groups prepared from the aforementionedfunctional groups using neutralizing agents and/or quaternizing agentsknown to those skilled in the art—such as primary, secondary, tertiaryand/or quaternary ammonium groups, tertiary sulfonium groups and/orquaternary phosphonium groups, more particularly quaternary ammoniumgroups and tertiary sulfonium groups.

As is well known, the functional groups for anionic modification are,for example, carboxylic acid, sulfonic acid and/or phosphonic acidgroups, more particularly carboxylic acid groups (functional groupswhich can be converted to anionic groups by neutralizing agents), andalso anionic groups—groups prepared from the aforementioned functionalgroups using neutralizing agents known to the skilled person—such ascarboxylate, sulfonate and/or phosphonate groups.

The functional groups for nonionic hydrophilic modification arepreferably poly(oxyalkylene) groups, more particularly poly(oxyethylene)groups.

The ionically hydrophilic modifications can be introduced into thepolyurethane resin through monomers which contain the ionic orpotentially ionic groups. The nonionic modifications are introduced, forexample, through the incorporation of poly(ethylene) oxide polymers aslateral or terminal groups in the polyurethane molecules. Thehydrophilic modifications are introduced, for example, via compoundswhich contain at least one group reactive toward isocyanate groups,preferably at least one hydroxyl group. The ionic modification can beintroduced using monomers which, as well as the modifying groups,contain at least one hydroxyl group. To introduce the nonionicmodifications, preference is given to using the polyether diols and/oralkoxypoly(oxyalkylene) alcohols known to those skilled in the art.

Preference is given to adding at least one organic solvent to theinitially charged polyurethane dispersion, said organic solventpreferably being miscible in any ratio with water and in any ratio withthe mixture of olefinically unsaturated monomers. Suitable organicsolvents are N-methylpyrrolidone, N-ethylpyrrolidone and ether alcohols,such as methoxypropanol in particular, though it should be noted thatpyrrolidone-based solvents may be dispensed with for environmentalreasons alone. However, the amount of the organic solvent is selectedsuch that the aqueous character of the dispersion is conserved.

In the second preparation step, a polymerization of a mixture ofolefinically unsaturated monomers in the presence of a polyurethane isconducted by the methods of what is called free-radical emulsionpolymerization in the presence of at least one polymerization initiator.

The polymerization initiator used has to be a water-soluble initiator.Examples of suitable initiators are potassium peroxodisulfate, sodiumperoxodisulfate or ammonium peroxodisulfate, and also hydrogen peroxide,tert-butyl hydroperoxide, 2,2′-azobis(2-amidoisopropane)dihydrochloride, 2,2′-azobis(N,N′-dimethylenisobutyramidine)dihydrochloride or 2,2′-azobis(4-cyano)pentanoic acid. The initiatorsare used either alone or in a mixture, for example mixtures of hydrogenperoxide and sodium persulfate.

The known redox initiator systems can also be used as polymerizationinitiators. Such redox initiator systems comprise at least oneperoxide-containing compound in combination with a redox coinitiator,for example reducing sulfur compounds, for example bisulfites, sulfites,thiosulfates, dithionites and tetrathionates of alkali metals andammonium compounds, sodium hydroxymethanesulfinate dihydrate and/orthiourea. For instance, it is possible to use combinations ofperoxodisulfates with alkali metal or ammonium hydrogensulfites, forexample ammonium peroxodisulfate and ammonium disulfite. The weightratio of peroxide-containing compounds to the redox coinitiators ispreferably 50:1 to 0.05:1. In combination with the initiators or theredox initiator systems, it is additionally possible to use transitionmetal catalysts, for example iron salts, nickel salts, cobalt salts,manganese salts, copper salts, vanadium salts or chromium salts, such asiron(II) sulfate, cobalt(II) chloride, nickel(II) sulfate, copper(I)chloride, manganese(II) acetate, vanadium(III) acetate, manganese(II)chloride. Based on the monomers, these transition metal salts aretypically used in amounts of 0.1 to 1000 ppm. For instance, it ispossible to use combinations of hydrogen peroxide with iron(II) salts,for example 0.5 to 30% hydrogen peroxide and 0.1 to 500 ppm of Mohr'ssalt.

The initiators are preferably used in an amount of 0.05 to 20% byweight, preferably 0.05 to 10%, more preferably of 0.1 to 5% by weight,based on the total weight of the olefinically unsaturated monomers usedfor polymerization. The terms “total amount” and “total weight” areequivalent. The result of the use of the water-soluble initiator is thatolefinically unsaturated monomers which are added to the aqueousdispersion initially charged can react immediately to give oligomers.These oligomers have a lesser tendency to penetrate into thepolyurethane particles of the dispersion initially charged than thesmaller monomers.

The polymerization is appropriately conducted, for example, at atemperature of greater than 0 to 160° C., preferably 60 to 95° C.

Preference is given to working under exclusion of oxygen, preferably ina nitrogen stream. In general, the polymerization is performed atstandard pressure, but it is also possible to employ lower pressures orhigher pressures, especially when polymerization temperatures above theboiling point of the monomers and/or of the organic solvents areemployed.

The copolymers (CP) for use in accordance with the invention areprepared by free-radical aqueous emulsion polymerization, in which casesurfactants or protective colloids can be added to the reaction medium.A list of suitable emulsifiers and protective colloids is given, forexample, in Houben Weyl, Methoden der organischen Chemie [Methods ofOrganic Chemistry], volume XIV/1 Makromolekulare Stoffe [MacromolecularSubstances], Georg Thieme Verlag, Stuttgart 1961, p. 411 ff.

An important factor for the preparation of the aqueous dispersions foruse in accordance with the invention, comprising the copolymer (CP), isthe control of the conditions of the polymerization reaction of themixture of olefinically unsaturated monomers in the presence of thepolyurethane. This is conducted in the manner of what is called a“starve feed”, “starve fed” or “starved feed” polymerization.

A starved feed polymerization in the context of the present invention isconsidered to be an emulsion polymerization in which the content ofresidual monomers in the reaction solution is minimized during thereaction time, meaning that the metered addition of the olefinicallyunsaturated monomers is effected in such a way that a concentration of6.0% by weight, preferably 5.0% by weight, more preferably 4.0% byweight, particularly advantageously 3.5% by weight, based in each caseon the total amount of olefinically unsaturated monomers used forpolymerization, is not exceeded over the entire reaction time. In thiscontext, further preference is given to concentration ranges of theolefinically unsaturated monomers of 0.01 to 6.0% by weight, preferably0.02 to 5.0% by weight, more preferably 0.03 to 4.0% by weight,especially 0.05 to 3.5% by weight, based in each case on the totalamount of olefinically unsaturated monomers used for polymerization. Forexample, the highest proportion (or concentration) detectable during thereaction may be 0.5% by weight, 1.0% by weight, 1.5% by weight, 2.0% byweight, 2.5% by weight or 3.0% by weight, while all further valuesdetected are then below the values specified here. The term“concentration” in this context is thus obviously equivalent to the term“proportion”.

The concentration of the monomers in the reaction solution, referred tohereinafter as free monomers, can be controlled in various ways.

One way of minimizing the concentration of the free monomers is toselect a very low metering rate for the mixture of olefinicallyunsaturated monomers. When the rate of metered addition is so low thatall monomers can react very quickly as soon as they are in the reactionsolution, it is possible to ensure that the concentration of the freemonomers is minimized.

As well as the metering rate, it is important that sufficient freeradicals are always present in the reaction solution, so that themonomers metered in can each be reacted very rapidly. For this purpose,reaction conditions should preferably be selected such that theinitiator feed is already commenced prior to commencement of the meteredaddition of the olefinically unsaturated monomers. Preferably, themetered addition is commenced at least 5 minutes beforehand, morepreferably at least 10 minutes beforehand. Preferably at least 10% byweight of the initiator, more preferably at least 20% by weight, mostpreferably at least 30% by weight of the initiator, based in each caseon the total amount of initiator, are added prior to commencement of themetered addition of the olefinically unsaturated monomers.

The amount of initiator is an important factor for the sufficientpresence of free radicals in the reaction solution. The amount ofinitiator should be selected such that sufficient free radicals areavailable at any time, so that the monomers metered in can react. If theamount of initiator is increased, it is also possible to react greateramounts of monomers at the same time.

A further factor which can determine the reaction rate is the structureof the monomers, i.e. particularly the structural properties thereof andthe reactivity which derives therefrom.

The concentration of the free monomers can thus be controlled throughthe interplay of the amount of initiator, rate of initiator addition,rate of monomer addition, and through the choice of monomers. Both theslowing of the metered addition and the increase in the amount ofinitiator, and also the early commencement of the addition of theinitiator, serve the particular aim of keeping the concentration of thefree monomers below the abovementioned limits.

The concentration of the monomers in the reaction solution can bedetermined by gas chromatography at any juncture in the reaction.Typical parameters for the gas chromatography determination are asfollows: 50 m silica capillary column with polyethylene glycol phase or50 m silica capillary column with polydimethylsiloxane phase, heliumcarrier gas, split injector 150° C., oven temperature 40 to 220° C.,flame ionization detector, detector temperature 275° C., internalstandard: isobutyl acrylate. In the context of the present invention,the concentration of the monomers is preferably determined by gaschromatography, especially while observing the abovementionedparameters.

Should this analysis determine a concentration of free monomers close tothe limit for the starved feed polymerization, for example because of ahigh proportion of olefinically unsaturated monomers having a lowreactivity, the abovementioned parameters can be utilized to control thereaction. In this case, for example, the metering rate of the monomerscan be reduced and/or the amount of initiator can be increased.

Suitable olefinically unsaturated monomers may be mono- orpolyolefinically unsaturated. Preferably, at least one monoolefinicallyunsaturated and at least one polyolefinically unsaturated monomer arepresent.

Examples of suitable monoolefinically unsaturated monomers includevinylic monoolefinically unsaturated monomers, such as especially(meth)acrylate-based monoolefinically unsaturated monomers and allylcompounds. Examples are also alpha,beta-unsaturated carboxylic acids.Preference is given to using at least, but not necessarily exclusively,(meth)acrylate-based monoolefinically unsaturated monomers.

The (meth)acrylate-based, monoolefinically unsaturated monomers may, forexample, be (meth)acrylic acid and esters, nitriles or amides of(meth)acrylic acid.

Preference is given to esters of (meth)acrylic acid having anon-olefinically unsaturated R radical.

The R radical may be aliphatic or aromatic. The R radical is preferablyaliphatic.

The R radical may, for example, be an alkyl radical, or containheteroatoms. Examples of R radicals containing heteroatoms are ethers.Preference is given to using at least, but not necessarily exclusively,monomers in which the R radical is an alkyl radical.

If R is an alkyl radical, it may, for example, be a linear, branched orcyclic alkyl radical. In all three cases, it may comprise unsubstitutedalkyl radicals or alkyl radicals substituted by functional groups. Thealkyl radical has preferably 1 to 20, more preferably 1 to 10, carbonatoms.

Particularly preferred monounsaturated esters of (meth)acrylic acidhaving an unsaturated alkyl radical are methyl (meth)acrylate, ethyl(meth)acrylate, propyl (meth)acylate, isopropyl (meth)acylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, amyl(meth)acrylate, hexyl (meth)acrylate, ethylhexyl (meth)acrylate,3,3,5-trimethylhexyl (meth)acrylate, stearyl (meth)acrylate, lauryl(meth)acrylate, cycloalkyl (meth)acrylates such as cyclopentyl(meth)acrylate, isobornyl (meth)acrylate and cyclohexyl (meth)acrylate,very particular preference being given to n- and tert-butyl(meth)acrylate and methyl methacrylate.

Suitable monounsaturated esters of (meth)acrylic acid having asubstituted alkyl radical may preferably be substituted by one or morehydroxyl groups.

Particularly preferred monounsaturated esters of (meth)acrylic acidhaving an alkyl radical substituted by one or more hydroxyl groups are2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate and4-hydroxybutyl (meth)acrylate.

Possible further vinylic monounsaturated monomers are monomers having anon-olefinically unsaturated R′ radical on the vinyl group.

The R′ radical may be aliphatic or aromatic, preference being given toaromatic radicals.

The R′ radical may be a hydrocarbyl radical, or contain heteroatoms.Examples of R′ radicals containing heteroatoms are ethers, esters,amide, nitriles and heterocycles. Preferably, the R′ radical is ahydrocarbyl radical. If R′ is a hydrocarbyl radical, it may besubstituted or unsubstituted by heteroatoms, preference being given tounsubstituted radicals. Preferably, the R′ radical is an aromatichydrocarbyl radical.

Particularly preferred further vinylic olefinically unsaturated monomersare vinylaromatic hydrocarbons, especially vinyltoluene,alpha-methylstyrene and especially styrene.

Further preferred monomers containing heteroatoms are olefinicallyunsaturated monomers such as acrylonitrile, methacrylonitrile,acrylamide, methacrylamide, N-dimethylacrylamide, vinyl acetate, vinylpropionate, vinyl chloride, N-vinylpyrrolidone, N-vinylcaprolactam,N-vinylformamide, N-vinylimidazole and N-vinyl-2-methylimidazoline.

Examples of suitable polyolefinically unsaturated monomers includeesters of (meth)acrylic acid having an olefinically unsaturated R″radical, and allyl ethers of polyhydric alcohols.

The R″ radicals may, for example, be an allyl radical or a (meth)acrylicester radical.

Preferred polyolefinically unsaturated monomers are ethylene glycoldi(meth)acrylate, propylene 1,2-glycol di(meth)acrylate, propylene2,2-glycol di(meth)acrylate, butane-1,4-diol di(meth)acrylate, neopentylglycol di(meth)acrylate, 3-methylpentanediol di(meth)acrylate,diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate,tetraethylene glycol di(meth)acrylate, dipropylene glycoldi(meth)acrylate, tripropylene glycol di(meth)acrylate, hexanedioldi(meth)acrylate and allyl (meth)acrylate.

Preferred polyolefinically unsaturated compounds are also acrylic andmethacrylic esters of alcohols having more than two OH groups, forexample trimethylolpropane tri(meth)acrylate or glyceryltri(meth)acrylate, but also trimethylolpropane di(meth)acrylatemonoallyl ether, trimethylolpropane (meth)acrylate diallyl ether,pentaerythrityl tri(meth)acrylate monoallyl ether, pentaerythrityldi(meth)acrylate diallyl ether, pentaerythrityl (meth)acrylate triallylether, triallylsucrose, and pentaallylsucrose.

Particular preference is given to using allyl methacrylate as thepolyolefinically unsaturated monomer.

The mixture of the olefinically unsaturated monomers comprises at leastone polyolefinically unsaturated monomer. Preferably, the mixture of theolefinically unsaturated monomers also comprises one or moremonounsaturated esters of (meth)acrylic acid having an unsubstitutedalkyl radical.

Preferably, the mixture of the olefinically unsaturated monomerscontains 0.1 to 6.0 mol %, more preferably 0.1 to 2.0 mol %, mostpreferably 0.1 to 1.0 mol %, of polyolefinically unsaturated monomers.Preferably, the radical of the olefinically unsaturated monomers ismonounsaturated.

Preferably, the mixture of the olefinically unsaturated monomerscontains 0.1 to 6.0 mol %, more preferably 0.1 to 2.0 mol %, mostpreferably 0.1 to 2.0 mol %, of allyl methacrylate. More preferably,apart from allyl methacrylate, no further polyolefinically unsaturatedmonomers are present in the mixture.

Preferably, the mixture of olefinically unsaturated monomers containsless than 10.0% by weight, more preferably less than 5.0% by weight, ofvinylaromatic hydrocarbons, based on the total amount of olefinicallyunsaturated monomers used in the polymerization. Most preferably, novinylaromatic hydrocarbons are present in the mixture of theolefinically unsaturated monomers. It is especially preferable when lessthan 10.0% by weight, more preferably less than 5.0% by weight, based onthe total amount of olefinically unsaturated monomers used in thepolymerization, of olefinically unsaturated monomers having aromaticgroups is used. More particularly, no olefinically unsaturated monomershaving aromatic groups are present in the mixture of the olefinicallyunsaturated monomers.

It follows from this that the vinylaromatic hydrocarbons specified aboveas preferred, especially vinyltoluene, alpha-methylstyrene and styrene,are of course preferred only within the group of the monomers containingaromatic groups. In spite of this, these monomers are preferably notused in the context of the invention. Should the use of such monomersnevertheless be an option in the individual case, preference is given tousing the monomers containing aromatic groups designated as preferred.

In a preferred embodiment, the mixture of olefinically unsaturatedmonomers comprises:

-   -   98.0 to 99.5% by weight of one or more monounsaturated esters of        (meth)acrylic acid having unsubstituted alkyl radicals, where        the alkyl radicals preferably have 1 to 10 carbon atoms, and    -   0.5 to 2.0% by weight of one or more polyunsaturated esters of        (meth)acrylic acid,        based in each case on the total amount of olefinically        unsaturated monomers used in the polymerization.

Preference is given to adding at least one solvent to the mixture ofolefinically unsaturated monomers, said solvent preferably beingmiscible in any ratio with water and in any ratio with the mixture ofolefinically unsaturated monomers. Suitable organic solvents areN-methylpyrrolidone, N-ethylpyrrolidone and ether alcohols, such asmethoxypropanol in particular, though it should be noted thatpyrrolidone-based solvents may be dispensed with for environmentalreasons alone. However, the amount of the organic solvent is selectedsuch that the aqueous character of the dispersion ultimately obtained isconserved.

By virtue of the preparation process described, the copolymers in theaqueous dispersion of the invention especially have a core-shellstructure which can be achieved through the preparation processdescribed. This core-shell structure is characterized by a corecontaining at least one polyurethane, and a shell containing at leastone polymer which has been obtained by polymerization of olefinicallyunsaturated monomers.

The core-shell structure described is achieved through the specificreaction conditions of the starved feed polymerization. Over the entirereaction time, there are never any great amounts of olefinicallyunsaturated monomers, which could penetrate into the polyurethaneparticles, in the presence of the initially charged polyurethane. Thefree radicals provided by the water-soluble initiator, which are alwayspresent during the addition of monomer in the aqueous phase, formoligomers immediately on addition, which can no longer penetrate intothe polyurethane. These then polymerize on the surface of thepolyurethane.

In a preferred embodiment, the weight ratio of core to shell is 80:20 to20:80, more preferably 60:40 to 40:60. What is meant here is the ratioof the amounts of components used for production of core (step (i),polyurethane) and shell (step (ii), mixture of olefinically unsaturatedmonomers).

Preferably, the copolymers (CP) in the aqueous dispersion have aparticle size (z average) of 60 to 130 nm, more preferably of 70 to 115nm, measured by means of photon correlation spectroscopy with a MalvernNano S90 (from Malvern Instruments) at 25±1° C. The instrument, equippedwith a 4 mW He—Ne laser at a wavelength of 633 nm, covers a size rangefrom 1 to 3000 nm.

The copolymers (CP) may preferably be crosslinked. The gel content ofthe aqueous dispersion of the invention is preferably 40 to 97% byweight, more preferably 75 to 90% by weight, based in each case on thesolids of the dispersion.

The gel content can be determined gravimetrically by freeze-drying thedispersion, determining the total mass of the freeze-dried polymer(corresponds to the solids of the dispersion in the context ofdetermining the gel content), and then extracting the polymer in anexcess of tetrahydrofuran (ratio of tetrahydrofuran to freeze-driedpolymer=300:1) at 25° C. for 24 hours. The insoluble fraction is removedand dried in an air circulation oven at 50° C. for four hours.Thereafter, the dried, insoluble fraction is weighed and the quotient isformed with the total mass of the freeze-dried polymer. The valueobtained corresponds to the gel content.

The weight-average molar mass of the copolymers (CP) is preferably 3*10⁷g/mol to 8.5*10⁹ g/mol, it being possible to determine theweight-average molar mass by small-angle laser light scattering.

The acid number of the copolymers (CP) is preferably 0 to 220 mg KOH/gsolid resin, preferably 0 to 40 mg KOH/g solid resin, more preferably 0to 25 mg KOH/g solid resin. The OH number is preferably less than 70 mgKOH/g solid resin, preferably less than 20 mg KOH/g solid resin. Theterms “solid resin” and “solids” in relation to a polymer or adispersion of a polymer are equivalent. Thus, they refer moreparticularly to the solids or solid content of a polymer dispersion aselucidated below.

The acid number can be determined on the basis of DIN EN ISO 2114 inhomogeneous solution of THF/water (9 parts by volume of THF and 1 partby volume of distilled water) with ethanolic potassium hydroxidesolution.

The OH number can be determined on the basis of R.-P. Kruger, R. Gnauckand R. Algeier, Plaste and Kautschuk, 20, 274 (1982), by means of aceticanhydride in the presence of 4-dimethylaminopyridine as a catalyst in atetrahydrofuran (THF)/dimethylformamide (DMF) solution at roomtemperature, by fully hydrolyzing the excess of acetic anthydrideremaining after acetylation and conducting a potentiometricback-titration of the acetic anhydride with alcoholic potassiumhydroxide solution.

The aqueous dispersions of the at least one copolymer (CP) preferablyhave a solids content of 15 to 45% by weight, especially preferably 25to 35% by weight. Solids contents of this kind can be establishedwithout any problem through the use of appropriate amounts of organicsolvents and especially water in the course of preparation of thecopolymers and/or by appropriate dilution after the preparation.

The proportion of the copolymers (CP) is preferably in the range from0.5 to 15.0% by weight, more preferably 0.75 to 12.5% by weight,especially preferably 1.0 to 10.0% by weight, especially 1.5 to 7.5% byweight, based in each case on the total weight of the aqueous basecoatmaterial (b.2.1).

The basecoat material (b.2.1) for use in accordance with the inventionadditionally comprises at least one specific reaction product (R),preferably exactly one reaction product (R).

The reaction products are linear. Linear reaction products can inprinciple be obtained by the conversion of difunctional reactants, inwhich case the linkage of the reactants via reaction of the functionalgroups gives rise to a linear, i.e. catenated, structure. Thus, forexample, if the reaction product is a polymer, the backbone has a linearcharacter. If the reaction product is, for example, a polyester, thereactants used may be diols and dicarboxylic acids, in which case thesequence of ester bonds in the reaction product has linear character.Preferably, in the preparation of the reaction product (R), principallydifunctional reactants are thus used. Other reactants, such asmonofunctional compounds in particular, are accordingly used preferablyonly in minor amounts, if at all. Especially at least 80 mol %,preferably at least 90 mol % and most preferably exclusivelydifunctional reactants are used. If further reactants are used, theseare preferably selected exclusively from the group of the monofunctionalreactants. It is preferable, however, that exclusively difunctionalreactants are used.

Useful functional groups for the reactants include the functional groupsknown to the person skilled in the art in this context. The combinationsof reactants having appropriate functional groups which can be linked toone another and can thus serve for preparation of the reaction productare also known in principle. The same applies to the reaction conditionsnecessary for linkage. Preferred functional groups for the reactants arehydroxyl, carboxyl, imino, carbamate, allophanate, thio, anhydride,epoxy, isocyanate, methylol, methylol ether, siloxane and/or aminogroups, especially preferably hydroxyl and carboxyl groups. Preferredcombinations of functional groups which can be linked to one another arehydroxyl and carboxyl groups, isocyanate and hydroxyl groups, isocyanateand amino groups, epoxy and carboxyl groups and/or epoxy and aminogroups; in choosing the functional groups, it should be ensured that thehydroxyl functionality and acid number described below are obtained inthe reaction product. The linkage then gives rise to the linkage pointsknown to those skilled in the art, for example ester groups, urethanegroups and/or urea groups. Very particular preference is given to acombination of hydroxyl and carboxyl groups. In this embodiment, atleast one reactant thus has hydroxyl groups, and at least one furtherreactant carboxyl groups. Preference is given to using a combination ofdihydroxy-functional and dicarboxy-functional reactants. Conducting thereaction of these reactants in a manner known per se forms reactionproducts containing ester bonds.

The reaction product is hydroxy-functional. It is preferable that thereactants are converted in such a way that linear molecules which formhave two terminal hydroxyl groups. This means that one hydroxyl group ispresent at each of the two ends of these molecules.

The reaction product has an acid number of less than 20, preferably lessthan 15, especially preferably less than 10 and most preferably lessthan 5 mg KOH/g. Thus, it preferably has only a very small amount ofcarboxylic acid groups. Unless explicitly stated otherwise, the acidnumber in the context of the present invention is determined to DIN53402. Thus, it relates to the reaction product per se, i.e. to thesolids content (for determination of the solids content see below).

If reference is made in the context of the present invention to anofficial standard without reference to the official period of validity,this of course means the version of the standard current at the filingdate or, if no current version exists at this date, the last currentversion.

The hydroxyl functionality described, just like the low acid number, canbe obtained, for example, in a manner known per se by the use ofappropriate ratios of reactants having appropriate functional groups. Inthe preferred case that dihydroxy-functional and dicarboxy-functionalreactants are used in the preparation, an appropriate excess of thedihydroxy-functional component is thus used. In this context, thefollowing should additionally be explained. For purely statisticalreasons alone, a real reaction of course does not just give moleculeshaving, for example, the desired (di)hydroxyl functionality. However,the choice of appropriate conditions, for example an excess ofdihydroxy-functional reactants, and conducting the reaction until thedesired acid number is obtained, guarantee that the conversion productsor molecules which make up the reaction product are hydroxy-functionalat least on average. The person skilled in the art knows how to chooseappropriate conditions.

In the preparation of the reaction product, at least one compound (v)used or converted as a reactant has two functional groups (v.a) and analiphatic or araliphatic hydrocarbyl radical (v.b) which is arrangedbetween the two functional groups and has 12 to 70, preferably 22 to 55and more preferably 30 to 40 carbon atoms. The compounds (v) thusconsist of two functional groups and the hydrocarbyl radical. Usefulfunctional groups of course include the above-described functionalgroups, especially hydroxyl and carboxyl groups. Aliphatic hydrocarbylradicals are known to be acyclic or cyclic, saturated or unsaturated,nonaromatic hydrocarbyl radicals. Araliphatic hydrocarbyl radicals arethose which contain both aliphatic and aromatic structural units.

The number-average molecular weight of the reaction products may varywidely and is, for example, from 600 to 40 000 g/mol, especially from800 to 10 000 g/mol, most preferably from 1200 to 5000 g/mol. Unlessexplicitly indicated otherwise, the number-average molecular weight inthe context of the present invention is determined by means of vaporpressure osmosis. Measurement was effected using a vapor pressureosmometer (model 10.00 from Knauer) on concentration series of thecomponent under investigation in toluene at 50° C., with benzophenone ascalibration substance for determination of the experimental calibrationconstant of the instrument employed (in accordance with E. Schroder, G.Müller, K.-F. Arndt, “Leitfaden der Polymercharakterisierung”,Akademie-Verlag, Berlin, pp. 47-54, 1982, in which benzil was used ascalibration substance). Preferred compounds (v) are dimer fatty acids,or are present in dimer fatty acids. In the preparation of the reactionproducts (R), dimer fatty acids are thus used preferably, but notexclusively, as compound (v). Dimer fatty acids (also long known asdimerized fatty acids or dimer acids) are generally, and especially inthe context of the present invention, mixtures prepared byoligomerization of unsaturated fatty acids. They are preparable, forexample, by catalytic dimerization of unsaturated plant fatty acids, thestarting materials used more particularly being unsaturated C₁₂ to C₂₂fatty acids. The bonds are formed principally by the Diels-Aldermechanism, and the result, depending on the number and position of thedouble bonds in the fatty acids used to prepare the dimer fatty acids,is mixtures of principally dimeric products having cycloaliphatic,linear aliphatic, branched aliphatic, and also C₆ aromatic hydrocarbongroups between the carboxyl groups. Depending on mechanism and/or anysubsequent hydrogenation, the aliphatic radicals may be saturated orunsaturated, and the fraction of aromatic groups may also vary. Theradicals between the carboxylic acid groups then contain, for example,24 to 44 carbon atoms. For the preparation, fatty acids having 18 carbonatoms are used with preference, and so the dimeric product has 36 carbonatoms. The radicals which join the carboxyl groups of the dimer fattyacids preferably have no unsaturated bonds and no aromatic hydrocarbonradicals.

In the context of the present invention, C₁₈ fatty acids are thus usedwith preference in the preparation. Particular preference is given tothe use of linolenic, linoleic and/or oleic acid.

Depending on the reaction regime, the above-identified oligomerizationgives rise to mixtures comprising primarily dimeric molecules, but alsotrimeric molecules and monomeric molecules and other by-products.Purification is typically effected by distillation. Commercial dimerfatty acids generally contain at least 80% by weight of dimericmolecules, up to 19% by weight of trimeric molecules, and not more than1% by weight of monomeric molecules and of other by-products.

Preference is given to using dimer fatty acids which consist to anextent of at least 90% by weight, preferably to an extent of at least95% by weight, most preferably at least to an extent of 98% by weight,of dimeric fatty acid molecules.

In the context of the present invention, preference is given to usingdimer fatty acids which consist of at least 90% by weight of dimericmolecules, less than 5% by weight of trimeric molecules, and less than5% by weight of monomeric molecules and other by-products. Particularpreference is given to the use of dimer fatty acids which consist of 95to 98% by weight of dimeric molecules, less than 5% by weight oftrimeric molecules, and less than 1% by weight of monomeric moleculesand of other by-products. Likewise used with particular preference aredimer fatty acids consisting of at least 98% by weight of dimericmolecules, less than 1.5% by weight of trimeric molecules, and less than0.5% by weight of monomeric molecules and other by-products. Thefractions of monomeric, dimeric, and trimeric molecules and of otherby-products in the dimer fatty acids can be determined, for example, bymeans of gas chromatography (GC). In that case, prior to the GCanalysis, the dimer fatty acids are converted to the correspondingmethyl esters via the boron trifluoride method (cf. DIN EN ISO 5509) andthen analyzed by means of GC.

A fundamental identifier of “dimer fatty acids” in the context of thepresent invention, therefore, is that their preparation involves theoligomerization of unsaturated fatty acids. This oligomerization givesrise principally, in other words to an extent preferably of at least 80%by weight, more preferably to an extent of at least 90% by weight, evenmore preferably to an extent of at least 95% by weight and moreparticularly to an extent of at least 98% by weight, to dimericproducts. The fact that the oligomerization thus gives rise topredominantly dimeric products containing exactly two fatty acidmolecules justifies this designation, which is commonplace in any case.An alternative expression for the relevant term “dimer fatty acids”,therefore, is “mixture comprising dimerized fatty acids”. The use ofdimeric fatty acids thus automatically implements the use ofdifunctional compounds (v). This also justifies the statement, chosen inthe context of the present invention, that dimer fatty acids arepreferably used as compound (v). This is because compounds (v) areapparently the main constituent of the mixtures referred to as dimerfatty acids. Thus, if dimer fatty acids are used as compounds (v), thismeans that these compounds (v) are used in the form of correspondingmixtures with above-described monomeric and/or trimeric molecules and/orother by-products.

The dimer fatty acids to be used can be obtained as commercial products.Examples include Radiacid 0970, Radiacid 0971, Radiacid 0972, Radiacid0975, Radiacid 0976, and Radiacid 0977 from Oleon, Pripol 1006, Pripol1009, Pripol 1012, and Pripol 1013 from Croda, Empol 1008, Empol 1061,and Empol 1062 from BASF, and Unidyme 10 and Unidyme TI from ArizonaChemical.

Further preferred compounds (v) are dimer diols, or are present in dimerdiols. Dimer diols have long been known and are also referred to in thescientific literature as dimeric fatty alcohols. These are mixtureswhich are prepared, for example, by oligomerization of unsaturated fattyacids or esters thereof and subsequent hydrogenation of the acid orester groups, or by oligomerization of unsaturated fatty alcohols. Thestarting materials used may be unsaturated C₁₂ to C₂₂ fatty acids oresters thereof, or unsaturated C₁₂ to C₂₂ fatty alcohols. Thehydrocarbyl radicals which connect the hydroxyl groups in the dimerdiols are defined in the same way as the hydrocarbyl radicals whichdivide the carboxyl groups of the dimer fatty acids.

For example, DE-11 98 348 describes the preparation thereof bydimerization of unsaturated fatty alcohols with basic alkaline earthmetal compounds at more than 280° C.

They can also be prepared by hydrogenation of dimer fatty acids and/oresters thereof as described above, according to German AuslegeschriftDE-B-17 68 313. Under the conditions described therein, not only are thecarboxyl groups of the fatty acids hydrogenated to hydroxyl groups, butany double bonds still present in the dimer fatty acids or estersthereof are also partly or fully hydrogenated. It is also possible toconduct the hydrogenation in such a way that the double bonds are fullyconserved during the hydrogenation. In this case, unsaturated dimerdiols are obtained. Preferably, the hydrogenation is conducted in such away that the double bonds are very substantially hydrogenated.

Another way of preparing dimer diols involves dimerizing unsaturatedalcohols in the presence of siliceous earth/alumina catalysts and basicalkali metal compounds according to international application WO91/13918.

Irrespective of the processes described for preparation of the dimerdiols, preference is given to using those dimer diols which have beenprepared from C₁₈ fatty acids or esters thereof, or C₁₈ fatty alcohols.In this way, predominantly dimer diols having 36 carbon atoms areformed.

Dimer diols which have been prepared by the abovementioned industrialprocesses always have varying amounts of trimer triols andmonofunctional alcohols. In general, the proportion of dimeric moleculesis more than 70% by weight, and the remainder is trimeric molecules andmonomeric molecules. In the context of the invention, it is possible touse either these dimer diols or purer dimer diols having more than 90%by weight of dimeric molecules. Particular preference is given to dimerdiols having more than 90 to 99% by weight of dimeric molecules, andpreference is given in turn among these to those dimer diols whosedouble bonds and/or aromatic radicals have been at least partly or fullyhydrogenated. An alternative expression for the relevant term “dimerdiols” is thus “mixture comprising dimers preparable by dimerization offatty alcohols”. The use of dimer diols thus automatically implementsthe use of difunctional compounds (v). This also justifies thestatement, chosen in the context of the present invention, that dimerdiols are used as compound (v). This is because compounds (v) areapparently the main constituent of the mixtures referred to as dimerdiols. Thus, if dimer diols are used as compounds (v), this means thatthese compounds (v) are used in the form of corresponding mixtures withabove-described monomeric and/or trimeric molecules and/or otherby-products.

Preferably, the mean hydroxyl functionality of the dimer diols should be1.8 to 2.2.

In the context of the present invention, particular preference istherefore given to using those dimer diols which can be prepared byhydrogenation from the above-described dimer fatty acids. Veryparticular preference is given to those dimer diols which consist of≧90% by weight of dimeric molecules, ≦5% by weight of trimericmolecules, and ≦5% by weight of monomeric molecules and of otherby-products, and/or have a hydroxyl functionality of 1.8 to 2.2.Particular preference is given to the use of those diols which can beprepared by hydrogenation from dimer fatty acids which consist of 95 to98% by weight of dimeric molecules, less than 5% by weight of trimericmolecules, and less than 1% by weight of monomeric molecules and ofother by-products. Particular preference is likewise given to the use ofthose diols which can be prepared by hydrogenation from dimer fattyacids which consist of ≧98% by weight of dimeric molecules, ≦1.5% byweight of trimeric molecules, and ≦0.5% by weight of monomeric moleculesand of other by-products.

Dimer fatty acids which can be used to prepare the dimer diols contain,as already described above, according to the reaction regime, bothaliphatic and possibly aromatic molecular fragments. The aliphaticmolecular fragments can be divided further into linear and cyclicfragments, which in turn may be saturated or unsaturated. Throughhydrogenation, the aromatic and the unsaturated aliphatic molecularfragments can be converted to corresponding saturated aliphaticmolecular fragments. The dimer diols usable as component (v) mayaccordingly be saturated or unsaturated. The dimer diols are preferablyaliphatic, especially aliphatic and saturated.

In the context of the present invention, preference is given to usingthose dimer diols which can be prepared by hydrogenation of thecarboxylic acid groups of preferably saturated aliphatic dimer fattyacids.

Particular preference is given to the use of those diols which can beprepared by hydrogenation from dimer fatty acids which consist of 98% byweight of dimeric molecules, ≦1.5% by weight of trimeric molecules, and0.5% by weight of monomeric molecules and of other by-products.

More preferably, the dimer diols have a hydroxyl number of 170 to 215 mgKOH/g, even more preferably of 195 to 212 mg KOH/g and especially 200 to210 mg KOH/g, determined by means of DIN ISO 4629. More preferably, thedimer diols have a viscosity of 1500 to 5000 mPas, even more preferably1800 to 2800 mPas (25° C., Brookfield, ISO 2555).

Dimer diols for use with very particular preference include thecommercial products Pripol® 2030 and especially Priopol® 2033 fromUniqema, or Sovermol® 908 from BASF.

Preferred reaction products (R) are preparable by reaction of dimerfatty acids with aliphatic, araliphatic or aromatic dihydroxy-functionalcompounds. Aliphatic compounds are nonaromatic organic compounds. Theymay be linear, cyclic or branched. Possible examples of compounds arethose which consist of two hydroxyl groups and an aliphatic hydrocarbylradical. Also possible are compounds which, as well as the oxygen atomspresent in the two hydroxyl groups, contain further heteroatoms such asoxygen or nitrogen, especially oxygen, for example in the form oflinking ether and/or ester bonds. Araliphatic compounds are those whichcontain both aliphatic and aromatic structural units. It is preferable,however, that the reaction products (R) are prepared by reaction ofdimer fatty acids with aliphatic dihydroxy-functional compounds.

The aliphatic, araliphatic or aromatic dihydroxy-functional compoundspreferably have a number-average molecular weight of 120 to 6000 g/mol,especially preferably of 200 to 4500 g/mol.

The statement of a number-average molecular weight thus implies thatpreferred dihydroxy-functional compounds are mixtures of various largedihydroxy-functional molecules. The dihydroxy-functional compounds arepreferably polyether diols, polyester diols or dimer diols.

It is preferable in the context of the present invention that the dimerfatty acids and the aliphatic, araliphatic and/or aromatic, preferablyaliphatic, dihydroxy-functional compounds are reacted with one anotherin a molar ratio of 0.7/2.3 to 1.6/1.7, preferably of 0.8/2.2 to 1.6/1.8and most preferably of 0.9/2.1 to 1.5/1.8. As a result of the excess ofhydroxyl groups, hydroxy-functional reaction products additionallyhaving a low acid number are thus obtained. Through the level of theexcess, it is possible to control the molecular weight of the reactionproduct. If only a small excess of the hydroxy-functional reactant isused, the result is correspondingly longer-chain products, since only inthat case is a substantial conversion of the acid groups presentguaranteed. In the case of a higher excess of the hydroxy-functionalreactant, the result is correspondingly shorter-chain reaction products.The number-average molecular weight of the reaction products is ofcourse also influenced by the molecular weight of the reactants, forexample the preferably aliphatic dihydroxy-functional compounds. Thenumber-average molecular weight of the preferred reaction products mayvary widely and is, for example, from 600 to 40 000 g/mol, especiallyfrom 800 to 10 000 g/mol, most preferably from 1200 to 5000 g/mol.

The preferred reaction products can thus also be described as linearblock-type compounds A-(B-A)_(n). In that case, at least one type ofblocks is based on a compound (v). Preferably, the B blocks are based ondimer fatty acids, i.e. compounds (v). The A blocks are preferably basedon aliphatic dihydroxy-functional compounds, especially preferably onaliphatic polyether diols, polyester diols or dimer diols. In the lattercase, the respective reaction product is thus based exclusively oncompounds (v) joined to one another.

Very particularly preferred reaction products are preparable by reactionof dimer fatty acids with at least one aliphatic dihydroxy-functionalcompound of the general structural formula (I):

where R is a C₃ to C₆ alkylene radical and n is correspondingly selectedsuch that the compound of the formula (I) has a number-average molecularweight of 120 to 6000 g/mol, the dimer fatty acids and the compounds ofthe formula (I) are used in a molar ratio of 0.7/2.3 to 1.6/1.7, and theresulting reaction product has a number-average molecular weight of 600to 40 000 g/mol and an acid number of less than 10 mg KOH/g.

In a very particularly preferred embodiment, n is thus selected heresuch that the compound of the formula (I) has a number-average molecularweight of 450 to 2200 g/mol, especially 800 to 1200 g/mol. R ispreferably a C₃ or C₄ alkylene radical. It is more preferably anisopropylene radical or a tetramethylene radical. Most preferably, thecompound of the formula (I) is polypropylene glycol orpolytetrahydrofuran. The dimer fatty acids and the compounds of theformula (I) are used here preferably in a molar ratio of 0.7/2.3 to1.3/1.7. In this embodiment, the resulting reaction product has anumber-average molecular weight of 1500 to 5000 g/mol, preferably 2000to 4500 g/mol and most preferably 2500 to 4000 g/mol.

Likewise very particularly preferred reaction products are preparable byreaction of dimer fatty acids with at least one dihydroxy-functionalcompound of the general structural formula (II):

whereR is a divalent organic radical comprising 2 to 10 carbon atoms,R¹ and R² are each independently straight-chain or branched alkyleneradicals having 2 to 10 carbon atoms,X and Y are each independently O, S or NR³ in which R³ is hydrogen or analkyl radical having 1 to 6 carbon atoms, andm and n are correspondingly selected such that the compound of theformula (II) has a number-average molecular weight of 450 to 2200 g/mol,in which components (a) and (b) are used in a molar ratio of 0.7/2.3 to1.6/1.7 and the resulting reaction product has a number-averagemolecular weight of 1200 to 5000 g/mol and an acid number of less than10 mg KOH/g,

In structural formula (II), R is a divalent organic radical comprising 2to 10 carbon atoms and preferably 2 to 6 carbon atoms. The R radicalmay, for example, be aliphatic, aromatic or araliphatic. The R radical,as well as carbon atoms and hydrogen atoms, may also containheteroatoms, for example 0 or N. The radical may be saturated orunsaturated. R is preferably an aliphatic radical having 2 to 10 carbonatoms, more preferably an aliphatic radical having 2 to 6 carbon atomsand most preferably an aliphatic radical having 2 to 4 carbon atoms. Forexample, the R radical is C₂H₄, C₃H₆, C₄H₈ or C₂H₄—O—C₂H₄.

R¹ and R² are each independently straight-chain or branched alkyleneradicals having 2 to 10 carbon atoms, preferably 2 to 6 carbon atoms andmore preferably 3 to 5 carbon atoms. These radicals preferably containonly carbon and hydrogen.

In the compounds of the structural formula (II), all n R¹ radicals andall m R² radicals may be identical. However, it is also possible thatdifferent kinds of R¹ and R² radicals are present. Preferably, all R¹and R² radicals are identical. With very particular preference, R¹ andR² are a C₄ or C₅ alkylene radical, especially a tetramethylene orpentamethylene radical. In a very particularly preferred embodiment ofthe present invention, both radicals, R¹ and R², are pentamethyleneradicals.

X and Y are each independently O, S or NR³ in which R³ is hydrogen or analkyl radical having 1 to 6 carbon atoms. Preferably, X and Y are eachindependently O or NR³; more preferably, they are each independently Oand NH; most preferably, X and Y are O.

The indices m and n are accordingly selected such that the compounds ofthe structural formula (II) have a number-average molecular weight of450 to 2200 g/mol, preferably 500 to 1400 g/mol, more preferably 500 to1200 g/mol.

The polyester polyols of the general structural formula (I) can beprepared by a first route, where compounds HX—R—YH act as startercompounds and the hydroxy-terminated polyester chains are polymerizedonto the starter compound by ring-opening polymerization of lactones ofthe hydroxycarboxylic acids HO—R¹—COOH and HO—R²—COOH. By a secondroute, it is of course also possible first to preparealpha-hydroxy-gamma-carboxy-terminated polyesters, for example byring-opening polymerization of lactones of the hydroxycarboxylic acidsHO—R¹—COOH and HO—R²—COOH, or by polycondensation of thehydroxycarboxylic acids HO—R¹—COOH and HO—R²—COOH. Thealpha-hydroxy-gamma-carboxy-terminated polyesters can then be reacted inturn with compounds HX—R—YH, by means of a condensation reaction, togive the polyester diols for use in accordance with the invention.

Corresponding processes are described, for example, in GermanOffenlegungsschrift 2234265 “Hydroxylendständige Polylactone”[Hydroxyl-terminal polylactones] from the applicant Stamicarbon N.V.

The dimer fatty acids and the compounds of the formula (II) are usedhere preferably in a molar ratio of 0.7/2.3 to 1.3/1.7. In thisembodiment, the resulting reaction product preferably has anumber-average molecular weight of 1200 to 5000 g/mol, preferably 1200to 4500 g/mol and most preferably 1200 to 4000 g/mol.

Likewise very particularly preferred reaction products (R) arepreparable by reaction of dimer fatty acids with dimer diols, in whichthe dimer fatty acids and dimer diols are used in a molar ratio of0.7/2.3 to 1.6/1.7 and the resulting reaction product has anumber-average molecular weight of 1200 to 5000 g/mol and an acid numberof less than 10 mg KOH/g.

Preferred dimer diols have already been described above. It ispreferable here that the dimer fatty acids and dimer diols are used in amolar ratio of 0.7/2.3 to 1.3/1.7. The resulting reaction product herepreferably has a number-average molecular weight of 1200 to 5000 g/mol,preferably 1300 to 4500 g/mol, and very preferably 1500 to 4000 g/mol.

It follows from the above statements that the reaction products (R) arepreparable by the exclusive use of compounds (v). For example, it ispossible to prepare the reaction products by the use of theabove-described preferred dimer fatty acids and dimer diols. Bothcompound classes are compounds (v), or both compound classes aremixtures comprising difunctional compounds (v). However, it is equallypossible to prepare reaction products (R) by the reaction of compounds(v), preferably dimer fatty acids, with other organic compounds,especially those of the structural formulae (I) and (II).

In the context of the present invention, it is preferable that 25 to 100mol % of at least one compound (v) are used in the preparation of thereaction products. If exclusively compounds (v) are used, it is evidentthat at least two compounds (v) are used.

The proportion of the reaction products (R) is preferably in the rangefrom 0.1 to 15% by weight, preferably 0.5 to 12% by weight, morepreferably 0.75 to 8% by weight, based in each case on the total weightof the aqueous basecoat material (b.2.1).

If the content of the reaction products (R) is below 0.1% by weight, itmay be the case that no further improvement is achieved in the stabilityto pinholes. If the content is more than 15% by weight, disadvantagesmay occur under some circumstances, for example incompatibility of saidreaction product in the aqueous coating composition. Suchincompatibility may be manifested, for example, in uneven leveling andalso in floating or settling.

The reaction product of the invention is generally sparingly soluble inaqueous systems. It is therefore preferably used directly in theproduction of the aqueous coating composition, and is not added to theotherwise finished coating composition only on completion of production.

The basecoat material (b.2.1) for use in accordance with the inventionadditionally comprises at least one specific polyurethane resin (X),preferably exactly one such polyurethane resin.

Polyurethane resins are known in principle; the preparation thereof andthe starting compounds usable in the preparation, especiallypolyisocyanates and polyols, which are also used for preparation of thepolyurethane resin (X), are described, for example, in WO 92/15405 A1,page 4 line 28 to page 10 line 32 and page 14 line 13 to page 15 line28, or else in DE 199 48 004 A1, page 4 line 19 to page 9 line 62 and/orelse further up in the description of the polyurethane used in step (i)of the preparation of the copolymer (CP).

It is preferable that the at least one polyurethane resin (X) iscarboxy-functional. It preferably has an acid number of 5 to 100 mgKOH/g, more preferably 7 to 50 mg KOH/g, especially preferably 15 to 35mg KOH/g. Especially the carboxylic acid groups present canhydrophilically stabilize the polyurethane resin (X), or thepolyurethane resin (X) has elevated dispersibility in aqueous media,such that it can be transferred into aqueous dispersions as describedbelow without adverse effects and hence can also be integrated into theaqueous coating composition of the invention. As is well known, suchcarboxylic acid groups, according to the pH or according to use ofneutralizing agents which are known in this context, may also be presentas ionic carboxylate groups. Carboxylic acid groups are thus among thefunctional groups which can be converted to ionic groups (potentiallyionic groups). Neutralization of the carboxyl groups is accomplishedusing, for example, ammonia, amines and/or amino alcohols, such as di-and triethylamine, dimethylaminoethanol, diisopropanolamine, morpholinesand/or N-alkylmorpholines.

The carboxylic acid groups are used in the preparation of thepolyurethane resin (X) through the use of corresponding startingcompounds for the preparation of these polyurethane resins, namelythrough carboxyl-containing compounds (x.1) containing at least onecarboxylic acid group and at least one functional group reactive towardisocyanate groups, preferably hydroxyl groups. In this way, it ispossible to incorporate the compound (x.1) into the polyurethane baseskeleton and, at the same time, to introduce carboxylic acid groups intothe polyurethane resin (X). Preferably, the compound (x.1) contains atleast one carboxylic acid group and at least two hydroxyl groups, mostpreferably one carboxylic acid group and two hydroxyl groups.

Useful compounds (x.1) include, if they contain carboxyl groups, forexample, polyether polyols and/or polyester polyols, especiallypolyester polyols. Polyester polyols of this kind are described, forexample, in DE 39 03 804 A1 and can be prepared in the manner known tothose skilled in the art. For example, said polyester polyols (x.1) canbe prepared via a standard polyester synthesis in organic solvents andin the presence of standard catalysts such as dibutyltin laurate, itbeing possible to use not only difunctional monomeric starting compoundstypically used, i.e. diols and dicarboxylic acids and correspondinganhydrides, but also trifunctional starting compounds such as triols,tricarboxylic acids and corresponding anhydrides, for exampletrimellitic anhydride, and dihydroxycarboxylic acids. In this way, it ispossible through the introduction of possible branching sites to preparepolyesters which may contain both hydroxyl groups and carboxylic acidgroups. Since a polyester, being a polymeric system, is always a mixtureof molecules of different size, it is apparent that the featuresspecified above, for example with regard to the existence of carboxylicacid groups or hydroxyl groups in polyesters (x.1), are regarded asstatistical mean values. Preferably, such compounds (x.1) havenumber-average molecular weights of 300 to 3000 g/mol. The preparationof such compounds (x.1) is possible through simple adjustment of, forexample, reaction conditions and ratios used for the starting materialsdescribed.

However, compounds (x.1) used are preferably low molecular weightcompounds having at least one carboxylic acid group and at least onefunctional group reactive toward isocyanate groups, preferably hydroxylgroups. In the context of the present invention, the expression “lowmolecular weight compounds”, as opposed to higher molecular weightcompounds, especially polymers, should be understood to mean those towhich a discrete molecular weight can be assigned, as preferablymonomeric compounds. A low molecular weight compound is thus, moreparticularly, not a polymer, since the latter are always a mixture ofmolecules and have to be described using mean molecular weights.Preferably, the term “low molecular weight compound” is understood tomean that corresponding compounds have a molecular weight of less than300 g/mol. Preference is given to the range from 100 to 200 g/mol.

Compounds (x.1) preferred in this context are, for example,monocarboxylic acids containing two hydroxyl groups, for exampledihydroxypropionic acid, dihydroxysuccinic acid and dihydroxybenzoicacid. Very particularly preferred compounds (x.1) arealpha,alpha-dimethylolalkanoic acids such as 2,2-dimethylolacetic acid,2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid and/or2,2-dimethylolpentanoic acid, especially 2,2-dimethylolpropionic acid.

The polyurethane resins (X) can be prepared by known and establishedmethods in bulk or solution, especially preferably by conversion of thestarting compounds used in typical organic solvents, such as methylethyl ketone, at elevated temperatures and optionally with use ofcatalysts typical for polyurethane preparation. Such catalysts are knownto those skilled in the art, one example being dibutyltin laurate.

It is preferable, however, that the preparation is effected attemperatures of not higher than 120° C., preferably of 30 to 100° C.,especially of 40 to 90° C. This has the advantage that the carboxylicacid groups present in the compounds (x.1) react only to a minor degree,if at all, with any polyols present to form ester bonds, such that thecarboxylic acid groups remain in their free form and hence can form thebasis of the carboxyl functionality of the polyurethane resin (X).However, the temperatures mentioned are in any case sufficient to permitan effective reaction of isocyanates with alcohols and amines. It islikewise preferable in this context when no catalysts usable forpolyurethane preparation, for example dibutyltin laurate, are used. Thisis because catalysts usable for polyurethane preparation are known togenerally catalyze ester formation too. Preferably, the polyurethaneresin, after the preparation, which preferably proceeds in organicsolvents, is transferred to an aqueous dispersion. This is preferablydone by adding neutralizing agents already mentioned above and addingwater, and distilling off the at least one organic solvent. The additionof water is conducted in such a way that the dispersions of thepolyurethane resin (X) have a solids content (nonvolatile content) ofpreferably 10 to 60% by weight, especially 15 to 45% by weight. Thepolyurethane resin (X) is preferably used in the coating composition ofthe invention in the form of such dispersions.

Preferably, the preparation of the polyurethane resins (X) proceeds insuch a way that difunctional starting compounds, preferably diols anddiisocyanates, and a component (x.1) are first reacted with one anotherto give a prepolymer still containing isocyanate groups, and the latteris then reacted with a starting compound having at least three hydroxylgroups, preferably exactly three hydroxyl groups. Preference is given tomonomeric triols, especially trimethylolethane, trimethylolpropane andglycerol. This is then followed, as described, by neutralization withpreferably tertiary amines and dispersion of the polyurethane resin (X)in water.

Preferably, in the first step, at least one polyester diol, at least onemonomeric diol, at least one monomeric compound (x.1) as described aboveand at least one polyisocyanate are reacted and then, in the secondstep, the product is reacted with at least one monomeric triol asdescribed above.

In the context of the present invention, it is preferable that at least70% by weight of the starting compounds used for preparation of thepolyurethane resin (X) are those which contain, as functionalpolymerizable groups (i.e. those suitable for formation of polymericbase skeletons), exclusively isocyanate groups, hydroxyl groups andcarboxylic acid groups, and also anhydride groups derived therefrom. Inthis way, and through the suitable selection of the reaction conditionsknown per se for preparation of polyurethane resins, it is possible toprepare resins in which the starting compounds are linked principallyvia urethane bonds (through reaction of isocyanate groups and hydroxylgroups) and the carboxylic acid groups remain in unconverted form in thepolymer for achievement of the acid number which is essential to theinvention. The polyurethane resin (B) is thus a resin in which, throughthe appropriate selection of starting compounds, the linkage of thesestarting compounds has been achieved principally through urethane bonds.Further functional polymerizable groups are known to those skilled inthe art. Ultimately, useful groups in this context are all the possiblegroups for preparation of polycondensation and polyaddition resins, andalso olefinically unsaturated groups as encountered in vinyl-containing,allyl-containing and acryloyl- and methacryloyl-containing startingcompounds for preparation of resins preparable by free-radicalpolymerization, such as (meth)acrylate (co)polymers. Examples includethio, N-methylolamino-N-alkoxymethylamino, imino, carbamate,allophanate, epoxy, methylol, methylol ether, siloxane, carbonate,amino, and beta-hydroxyalkylamide groups, and also vinyl, allyl,acryloyl and methacryloyl groups. It is preferable that at least 80% byweight, preferably at least 90% by weight and most preferably 100% byweight of the starting compounds used for preparation of thepolyurethane resins (X) contain exclusively isocyanate groups, hydroxylgroups and carboxylic acid groups as polymerizable groups. It is thusespecially preferable that the polyurethane resin (X) is not acopolymer.

The number-average molecular weight of the polyurethane resins (X) mayvary widely. It is preferably in the range from 1000 to 50 000 g/mol,especially 2000 to 30 000 g/mol.

The proportion of the polyurethane resins (X) is preferably in the rangefrom 0.1 to 15% by weight, preferably 0.5 to 10% by weight, morepreferably 0.75 to 8% by weight, based in each case on the total weightof the aqueous basecoat material (b.2.1).

The basecoat material (b.2.1) preferably comprise pigments, i.e. colorpigments and/or effect pigments. Such color pigments and effect pigmentsare known to those skilled in the art and are described, for example, inRömpp-Lexikon Lacke and Druckfarben, Georg Thieme Verlag, Stuttgart,N.Y., 1998, pages 176 and 451. The terms “coloring pigment” and “colorpigment” are interchangeable, just like the terms “visual effectpigment” and “effect pigment”.

Preferred effect pigments are, for example, platelet-shaped metal effectpigments such as lamellar aluminum pigments, gold bronzes, oxidizedbronzes and/or iron oxide-aluminum pigments, pearlescent pigments suchas pearl essence, basic lead carbonate, bismuth oxide chloride and/ormetal oxide-mica pigments and/or other effect pigments such asplatelet-shaped graphite, platelet-shaped iron oxide, multilayer effectpigments composed of PVD films and/or liquid crystal polymer pigments.Particular preference is given to platelet-shaped metal effect pigments,especially platelet-shaped aluminum pigments.

Typical color pigments especially include inorganic coloring pigmentssuch as white pigments such as titanium dioxide, zinc white, zincsulfide or lithopone; black pigments such as carbon black, ironmanganese black, or spinel black; chromatic pigments such as chromiumoxide, chromium oxide hydrate green, cobalt green or ultramarine green,cobalt blue, ultramarine blue or manganese blue, ultramarine violet orcobalt violet and manganese violet, red iron oxide, cadmiumsulfoselenide, molybdate red or ultramarine red; brown iron oxide, mixedbrown, spinel phases and corundum phases or chromium orange; or yellowiron oxide, nickel titanium yellow, chromium titanium yellow, cadmiumsulfide, cadmium zinc sulfide, chromium yellow or bismuth vanadate.

The proportion of the pigments may, for example, be within the rangefrom 0.5 to 30% by weight, preferably 1.5 to 20% by weight, morepreferably 2.0 to 15% by weight, based in each case on the total weightof the aqueous basecoat material (b.2.1).

The basecoat material (b.2.1) comprises, through the use of components(CP), (R) and (X), curable binders, especially physically and thermallycurable binders. A “binder” in the context of the present invention andin accordance with relevant DIN EN ISO 4618 is the nonvolatile componentof a coating composition, without pigments and fillers. Specific bindersare accordingly, for example, also standard coatings additives, thecopolymer (CP), the reaction product (R), the polyurethane resin (X) orfurther polymers usable as described below, and typical crosslinkingagents as described below. However, the expression is used hereinafter,merely for the sake of better clarity, principally in relation toparticular physically and thermally curable polymers, for exampleparticular polyurethanes, polyesters, polyacrylates and/or copolymers ofthe polymers mentioned.

The basecoat material (b.2.1) preferably also comprises at least onepolymer other than the copolymers (CP), the reaction products (R) andthe polyurethane resins (X) as a binder, especially at least one polymerselected from the group consisting of polyurethanes, polyesters,polyacrylates and/or copolymers of the polymers mentioned, especiallypolyurethane polyacrylates. Preferred polyurethane-polyacrylatecopolymers (acrylated polyurethanes) other than the copolymers (CP) andthe preparation thereof are described, for example, in WO 91/15528 A1,page 3 line 21 to page 20 line 33, and in DE 4437535 A1, page 2 line 27to page line 22. The polymers described as binders are preferablyhydroxy-functional. Preferably, the coating compositions of theinvention comprise, as well as the at least one copolymer (CP), the atleast one reaction product (R) and the at least one polyurethane resin(X), at least one polyurethane-polyacrylate copolymer other than thecopolymers (CP).

The proportion of the further polymers as a binder, preferably the atleast one polyurethane-polyacrylate copolymer other than the copolymers(CP), is preferably in the range from 0.5 to 20.0% by weight, morepreferably 1.0 to 15.0% by weight, especially preferably 1.5 to 10.0% byweight, based in each case on the total weight of the aqueous basecoatmaterial (b.2.1).

In addition, the basecoat material (b.2.1) preferably comprises at leastone typical crosslinking agent known per se. It preferably comprises, asa crosslinking agent, at least one aminoplast resin and/or a blockedpolyisocyanate, preferably an aminoplast resin. Among the aminoplastresins, melamine resins in particular are preferred.

The proportion of the crosslinking agents, especially aminoplast resinsand/or blocked polyisocyanates, more preferably aminoplast resins, amongthese preferably melamine resins, is preferably in the range from 0.5 to20.0% by weight, more preferably 1.0 to 15.0% by weight, especiallypreferably 1.5 to 10.0% by weight, based in each case on the totalweight of the aqueous basecoat material (b.2.1).

Preferably, the basecoat material (b.2.1) additionally comprises atleast one thickener. Suitable thickeners are inorganic thickeners fromthe group of the sheet silicates. Lithium-aluminum-magnesium silicatesare particularly suitable. As well as the organic thickeners, however,it is also possible to use one or more organic thickeners. These arepreferably selected from the group consisting of (meth)acrylicacid-(meth)acrylate copolymer thickeners, for example the commercialproduct Rheovis AS S130 (BASF), and of polyurethane thickeners, forexample the commercial product Rheovis PU 1250 (BASF). The thickenersused are different than the above-described polymers, for example thepreferred binders. Preference is given to inorganic thickeners from thegroup of the sheet silicates.

The proportion of the thickeners is preferably in the range from 0.01 to5.0% by weight, preferably 0.02 to 4% by weight, more preferably 0.05 to3.0% by weight, based in each case on the total weight of the aqueousbasecoat material (b.2.1).

In addition, the aqueous basecoat material (b.2.1) may also comprise atleast one additive. Examples of such additives are salts which can bebroken down thermally without residue or substantially without residue,resins as binders that are curable physically, thermally and/or withactinic radiation and are different than the polymers already mentioned,further crosslinking agents, organic solvents, reactive diluents,transparent pigments, fillers, dyes soluble in a molecular dispersion,nanoparticles, light stabilizers, antioxidants, deaerating agents,emulsifiers, slip additives, polymerization inhibitors, initiators offree-radical polymerizations, adhesion promoters, flow control agents,film-forming assistants, sag control agents (SCAs), flame retardants,corrosion inhibitors, waxes, siccatives, biocides, and flatting agents.

Suitable additives of the aforementioned kind are known, for example,from

-   -   German patent application DE 199 48 004 A1, page 14 line 4 to        page 17 line 5,    -   German patent DE 100 43 405 C1, column 5, paragraphs [0031] to        [0033].

They are used in the customary and known amounts. For example, theproportion thereof may be in the range from 1.0 to 20.0% by weight,based in each case on the total weight of the aqueous basecoat material(b.2.1).

The solids content of the basecoat materials may vary according to therequirements of the individual case. The solids content is guidedprimarily by the viscosity required for application, more particularlyfor spray application, and so may be adjusted by the skilled person onthe basis of his or her general art knowledge, optionally withassistance from a few exploratory tests.

The solids content of the basecoat materials (b.2.1) is preferably 5 to70% by weight, more preferably 8 to 60% by weight, most preferably 12 to55% by weight.

By solids content (nonvolatile fraction) is meant that weight fractionwhich remains as a residue on evaporation under specified conditions. Inthe present specification, the solids content is determined to DIN ENISO 3251. This is done by evaporating the basecoat material at 130° C.for 60 minutes.

Unless stated otherwise, this test method is likewise employed in order,for example, to find out or predetermine the proportion of variouscomponents of the basecoat material, for example of a polyurethaneresin, a copolymer (CP) or a crosslinking agent, in the total weight ofthe basecoat material. Thus, the solids content of a dispersion of apolyurethane resin, a copolymer (CP) or a crosslinking agent which is tobe added to the basecoat material is determined. By taking into accountthe solids content of the dispersion and the amount of the dispersionused in the basecoat material, it is then possible to ascertain or findout the proportion of the component in the overall composition.

The basecoat material (b.2.1) is aqueous. The expression “aqueous” isknown in this context to the skilled person. The phrase refers inprinciple to a basecoat material which is not based exclusively onorganic solvents, i.e., does not contain exclusively organic-basedsolvents as its solvents but instead, in contrast, includes asignificant fraction of water as solvent. “Aqueous” for the purposes ofthe present invention should preferably be understood to mean that thecoating composition in question, more particularly the basecoatmaterial, has a water fraction of at least 40% by weight, preferably atleast 45% by weight, very preferably at least 50% by weight, based ineach case on the total amount of the solvents present (i.e., water andorganic solvents). Preferably in turn, the water fraction is 40 to 95%by weight, more particularly 45 to 90% by weight, very preferably 50 to85% by weight, based in each case on the total amount of solventspresent.

The same definition of “aqueous” of course also applies to all furthersystems described in the context of the present invention, for exampleto the aqueous character of the electrocoat materials (e.1) or theaqueous character of the aqueous dispersions of the copolymers (CP).

The basecoat materials (b.2.1) used in accordance with the invention canbe produced using the mixing assemblies and mixing techniques that arecustomary and known for the production of basecoat materials.

At least one of the basecoat materials (b.2.2.x) used in the method ofthe invention has the features essential to the invention as describedfor the basecoat material (b.2.1). All the preferred embodiments andfeatures described within the description of the basecoat material(b.2.1) apply preferentially to at least one of the basecoat materials(b.2.2.x).

In the above-described preferred variant (a) of stage (2.2) of themethod of the invention, in which the two basecoat materials (b.2.2.x)used are identical, both basecoat materials (b.2.2.x) evidently have thefeatures essential to the invention as described for the basecoatmaterial (b.2.1). In this variant, the basecoat materials (b.2.2.x)preferably comprise effect pigments as described above, especiallylaminar aluminum pigments. Preferred proportions are 2 to 10% by weight,preferably 3 to 8° s by weight, based in each case on the total weightof the basecoat material. However, it may also comprise furtherpigments, i.e. particularly chromatic pigments.

In the above-described preferred variant (b) of stage (2.2) of themethod of the invention, a first basecoat material (b.2.2.a) ispreferably applied first, which can also be referred to as acolor-preparatory basecoat material. It serves as a primer for abasecoat film which then follows, and which can then optimally fulfillits function of imparting color and/or an effect.

In a first particular embodiment of variant (b), a color-preparatorybasecoat material of this kind is essentially free of chromatic pigmentsand effect pigments. More particularly, a basecoat material (b.2.2.a) ofthis kind contains less than 2% by weight, preferably less than 1% byweight, of chromatic pigments and effect pigments, based in each case onthe total weight of the aqueous basecoat material. It is preferably freeof such pigments. In this embodiment, the color-preparatory basecoatmaterial comprises preferably black and/or white pigments, especiallypreferably both kinds of these pigments. Preferably, it contains 5 to20% by weight, preferably 8 to 12% by weight, of white pigments and 0.05to 1% by weight, preferably 0.1 to 0.5% by weight, of black pigments,based in each case on the total weight of the basecoat material. Thegray color which results therefrom, which can be set at differentbrightness levels through the ratio of white and black pigments,constitutes an individually adjustable base for the basecoat buildupwhich then follows, such that the color and/or effect imparted by thebasecoat material buildup which follows can be manifested optimally. Thepigments are known to those skilled in the art and are also describedabove. A preferred white pigment here is titanium dioxide, a preferredblack pigment carbon black.

For the basecoat material for the second basecoat, or for the second andthird basecoats, within this embodiment of variant (b), the samepreferably applies as was stated for basecoat material (b.2.2.x)described in variant (a). More particularly, it preferably compriseseffect pigments. Both for the color-preparatory basecoat material(b.2.2.x) and for the second basecoat material (b.2.2.x) preferablycomprising effect pigments, the features essential to the invention asdescribed for the basecoat material (b.2.1) have to be fulfilled. Ofcourse, both basecoat materials (b.2.2.x) may also fulfill thesefeatures.

In a second particular embodiment of the present invention, it is alsopossible for the color-preparatory basecoat material (b.2.2.a) tocomprise chromatic pigments. This variant is an option especially whenthe resulting multicoat paint system is to have a highly chromatic hue,for example a very deep red or yellow. In that case, thecolor-preparatory basecoat material (b.2.2.a) contains, for example, aproportion of 2 to 6% by weight of chromatic pigments, especially redpigments are/or yellow pigments, preferably in combination with 3 to 15%by weight, preferably 4 to 10% by weight, of white pigments. The atleast one further basecoat material which is then applied subsequentlythen obviously likewise comprises the chromatic pigments described, suchthat the first basecoat material (b.2.2.a) again serves for colorpreparation. In this embodiment too, any individual basecoat material(b.2.2.x), a plurality thereof or each of them may be one which fulfillsthe features essential to the invention as described for the basecoatmaterial (b.2.1).

In the above-described preferred variant (c) of stage (2.2) of themethod of the invention too, any individual basecoat material (b.2.2.x),a plurality thereof or each of them may be one which fulfills thefeatures essential to the invention as described for the basecoatmaterial (b.2.1).

The method of the invention allows the production of multicoat paintsystems without a separate curing step. In spite of this, the employmentof the method according to the invention results in multicoat paintsystems having excellent impact resistance, especially stone-chipresistance.

The method described can in principle also be used for production ofmulticoat paint systems on nonmetallic substrates, for example plasticssubstrates. In that case, the basecoat material (b.2.1) or the firstbasecoat material (b.2.2.a) is applied to an optionally pretreatedplastics substrate, preferably directly to an optionally pretreatedplastics substrate.

The present invention is illustrated hereinafter by examples.

EXAMPLES A) Preparation of a Copolymer (CP) or of an Aqueous DispersionComprising Said Polymer

a) A dispersion of an alpha-methylstyryl-containing polyurethane wasprepared on the basis of the patent DE 19948004 B4, page 27, example 1,“Herstellung eines erfindungsgemäβen Polyurethans (B)” [“Preparation ofa polyurethane (B) of the invention”], except with additional use oftrimethylolpropane and with a solids content of the resulting dispersionof only 29% rather than 35.1% by weight. Based on the adduct (B2)mentioned in the patent DE 19948004 B4, preparation example 1, an adductwas prepared with monoethanolamine rather than with diethanolamine:

For this purpose, a reaction vessel equipped with stirrer, internalthermometer, reflux condenser and electrical heater was first initiallycharged, under nitrogen, with 200.0 parts by weight of methyl ethylketone, 800.0 parts by weight of N-methylpyrrolidone and 221.3 parts byweight of monoethanolamine (from BASF SE) at 20° C. To this mixture wereadded dropwise, over the course of one and a half hours, 778.7 parts byweight of 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene(TMI® (META) Unsaturated Aliphatic Isocyanate, from Cytec) having anisocyanate content of 20.4% by weight of isocyanate, such that thereaction temperature did not exceed 40° C. The resulting reactionmixture was stirred until no free isocyanate groups were detectable anylonger. Thereafter, the reaction mixture was stabilized with 200 ppm ofhydroquinone.

The theoretical solids content of the solution of the described adductthus-prepared was 50% by weight.

Then, in a further reaction vessel equipped with stirrer, internalthermometer, reflux condenser and electrical heater, 431.7 parts byweight of a linear polyester polyol and 69.7 parts by weight ofdimethylolpropionic acid (from GEO Specialty Chemicals) were dissolvedin 355.8 parts by weight of methyl ethyl ketone and 61.6 parts by weightof N-methylpyrrolidone under nitrogen. The linear polyester polyol hadbeen prepared beforehand from dimerized fatty acid (Pripol® 1012, fromUniqema), isophthalic acid (from BP Chemicals) and hexane-1,6-diol (fromBASF SE) (weight ratio of the starting materials:dimeric fatty acid toisophthalic acid to hexane-1,6-diol=54.00:30.02:15.98) and had ahydroxyl number of 73 mg KOH/g solids and a number-average molar mass of1379 g/mol. Added to the resulting solution at 45° C. were 288.6 partsby weight of isophorone diisocyanate (Basonat® I, from BASF SE) havingan isocyanate content of 37.75% by weight. After the exothermic reactionhad abated, the reaction mixture was heated gradually to 80° C. whilestirring. Stirring was continued at this temperature until theisocyanate content of the solution was constant at 3.2% by weight.Thereafter, the reaction mixture was cooled to 65° C., and 85.2 parts byweight of the above-described adduct were added together with 21.8 partsby weight of trimethylolpropane (from BASF SE). The resulting reactionmixture was stirred at 65° C. until the isocyanate content of thesolution had fallen to 1.0% by weight. Now 22.2% by weight of thediethanolamine (from BASF SE) were added and the content of isocyanategroups was monitored until no free isocyanate groups were detectable anylonger. The resulting dissolved polyurethane was admixed with 139.7parts by weight of methoxypropanol and 43.3 parts by weight oftriethylamine (from BASF SE). 30 minutes after the addition of amine,the temperature of the solution was lowered to 60° C., after which 1981parts by weight of deionized water were added while stirring over thecourse of 30 minutes. The methyl ethyl ketone was distilled out of theresulting dispersion at 60° C. under reduced pressure. Thereafter, anylosses of solvent and water were compensated for.

The dispersion of an alpha-methylstyryl-containing polyurethane thusobtained had a solids content of 29.0% by weight, the acid number was34.0 mg KOH/g solids, and the pH was 7.0 (measured at 23° C.)

b) To prepare the aqueous primary dispersion of the copolymer (CP) ofthe invention, under a nitrogen atmosphere, 1961.2 parts by weight ofthe alpha-methylstyryl-containing polyurethane dispersion according toa) were diluted with 40.0 parts by weight of methoxypropanol (0.07%based on polyurethane) and 686.5 parts by weight of deionized water, andheated to 80° C. After the reactor contents had been heated to 80° C.,0.6 part by weight of ammonium peroxodisulfate, dissolved in 35.7 partsby weight of deionized water, were introduced into the reactor understandard pressure. Subsequently, with continued stirring, a mixture of301.6 parts by weight of methyl methacrylate, 261.6 parts by weight ofn-butyl acrylate, 5.6 parts by weight of allyl methacrylate (0.87 mol %based on total vinyl monomer) and 134.9 parts by weight ofN-methylpyrrolidone was added homogeneously over the course of fivehours. With commencement of the addition of the monomer mixture, asolution of 1.1 parts by weight of ammonium peroxodisulfate in 71.3parts by weight of deionized water was likewise added within five hours.

During the free-radical polymerization, every 30 minutes, the content offree monomers was determined by means of gas chromatography (GC) (GC:once with 50 m silica capillary column with polyethylene glycol phaseand once with 50 m silica capillary column with polydimethylsiloxanephase, carrier gas: helium, split injector 150° C., oven temperature40-220° C., flame ionization detector, detector temperature 275° C.,internal standard: isobutyl acrylate), and the highest total monomercontent based on dispersion of 0.5% by weight was found after 30 min(3.1% by weight based on the total amount of olefinically unsaturatedmonomers used for polymerization). After the simultaneous end of themetered addition of monomer and initiator, the resulting reactionmixture was stirred at 80° C. for a further hour and then cooled to roomtemperature.

The resulting primary dispersion of the copolymer had a very goodstorage stability. The solids content thereof was 32.5% by weight, theacid number was 18.8 mg KOH/g solids, and the pH thereof was 7.0. Theparticle size (z average) by means of photon correlation spectroscopywas 96 nm. By means of gas chromatography (GC: once with 50 m silicacapillary column with polyethylene glycol phase and once with 50 msilica capillary column with polydimethylsiloxane phase, carrier gas:helium, split injector 250° C., oven temperature 40-220° C., flameionization detector, detector temperature 275° C., internal standard:n-propyl glycol), a content of 2.7% by weight of methoxypropanol and5.7% by weight of N-methylpyrrolidone was found.

After the extraction of the freeze-dried polymer by means oftetrahydrofuran, the gel content was found gravimetrically to be 80.3%by weight. For this purpose, the dispersion was freeze-dried and themass of the freeze-dried polymer was determined, and then the polymerwas extracted in an excess of tetrahydrofuran (ratio of tetrahydrofuranto freeze-dried copolymer=300:1) at 25° C. for 24 hours. The insolublecontent (gel content) was isolated, dried at 50° C. in an aircirculation oven for 4 hours, and then re-weighed.

B) Preparation of a Reaction Product (R)

In a 4 l stainless steel reactor equipped with anchor stirrer,thermometer, condenser, thermometer for overhead temperature measurementand water separator, 2000.0 g of linear diolic PolyTHF1000 (2 mol),579.3 g of dimer fatty acid (1 mol) and 51 g of cyclohexane were heatedto 100° C. in the presence of 2.1 g of di-n-butyltin oxide (Axion® CS2455, from Chemtura). Heating was continued gently until the onset ofthe condensation. With a maximum overhead temperature of 85° C., heatingwas then continued in steps up to 220° C. The progress of the reactionwas monitored via the determination of the acid number. When an acidnumber of 3 mg KOH/g was reached, cyclohexane still present was removedby vacuum distillation. A viscous resin was obtained.

Amount of condensate (water): 34.9 g

Acid number: 2.7 mg KOH/g

Solids content (60 min at 130° C.): 100.0%

Molecular weight (vapor pressure osmosis):

Mn: 2200 g/mol

Viscosity: 5549 mPas,

(measured at 23° C. using a rotational viscometer from Brookfield, modelCAP 2000+, spindle 3, shear rate: 1333 s⁻¹)

1. Production of a Non-Inventive Waterborne Basecoat Material 1

The components listed under “aqueous phase” in table A were stirredtogether in the order stated to form an aqueous mixture. The combinedmixture was then stirred for 10 minutes and adjusted, using deionizedwater and dimethylethanolamine, to a pH of 8 and to a spray viscosity of58 mPas under a shearing load of 1000 s⁻¹ as measured with a rotaryviscometer (Rheomat RM 180 instrument from Mettler-Toledo) at 23° C.

TABLE A Waterborne basecoat material 1 Component Aqueous phase Parts byweight 3% Na—Mg sheet silicate solution 14 Deionized water 16 Butylglycol 1.4 Polyester; prepared as per example D, column 2.3 16 lines37-59 of DE-A-4009858 3% by weight aqueous Rheovis ® AS S130 6 solution;rheological agent, available from BASF, in water TMDD (BASF) 1.6Melamine-formaldehyde resin (Cymel ® 1133 5.9 from Allnex) 10%dimethylethanolamine in water 0.4 Polyurethane dispersion - prepared asper WO 20 92/15405 (page 14 line 13 to page 15 line 13) 2-Ethylhexanol3.5 Triisobutyl phosphate 2.5 Nacure ® 2500 from King Industries 0.6White paste 24 Carbon black paste 1.8

Production of the Carbon Black Paste:

The carbon black paste was produced from 25 parts by weight of anacrylated polyurethane dispersion produced as per international patentapplication WO 91/15528, binder dispersion A, 10 parts by weight ofcarbon black, 0.1 part by weight of methyl isobutyl ketone, 1.36 partsby weight of dimethylethanolamine (10% in demineralized water), 2 partsby weight of a commercial polyether (Pluriol® P900 from BASF SE) and61.45 parts by weight of deionized water.

Production of the White Paste:

The white paste was produced from 43 parts by weight of an acrylatedpolyurethane dispersion produced as per international patent applicationWO 91/15528, binder dispersion A, 50 parts by weight of titanium rutile2310, 3 parts by weight of 1-propoxy-2-propanol and 4 parts by weight ofdeionized water.

2. Preparation of a Non-Inventive Waterborne Basecoat Material 2

The components listed under “aqueous phase” in table B were stirredtogether in the order stated to form an aqueous mixture. In the nextstep an organic mixture was prepared from the components listed under“organic phase”. The organic mixture was added to the aqueous mixture.The combined mixture was then stirred for 10 minutes and adjusted, usingdeionized water and dimethylethanolamine, to a pH of 8 and to a sprayviscosity of 58 mPas under a shearing load of 1000 s⁻¹ as measured witha rotary viscometer (Rheomat RM 180 instrument from Mettler-Toledo) at23° C.

TABLE B Waterborne basecoat material 2 Component Parts by weight Aqueousphase 3% Na—Mg sheet silicate solution 29.1 Deionized water 10.5 Butylglycol 4.1 Polyurethane-modified polyacrylate; prepared 2.8 as per page7 line 55 to page 8 line 23 of DE 4437535 A1 50% strength by weightsolution of Rheovis ® 0.2 PU 1250 (BASF), rheological agent Polyester;prepared as per example D, column 5.5 16 lines 37-59 of DE-A-4009858TMDD (BASF) 1.4 Melamine-formaldehyde resin (Luwipal 052 2.9 from BASFSE) 10% dimethylethanolamine in water 0.4 Polyurethane-based graftcopolymer; prepared 24.1 analogously to DE 19948004 - B4 (page 27,example 2), solids content adjusted to 32.5% by weight with waterIsopropanol 1.6 Isopar ® L from Exxon Mobil 2 BYK-347, Altana 0.6Pluriol ® P 900 from BASF SE 0.4 Tinuvin ® 384-2 from BASF SE 0.7Tinuvin 123 from BASF SE 0.4 Carbon black paste 0.4 Blue paste 1.5Organic phase Aluminum pigment, available from Altana- 3.8 Eckart Butylglycol 3.8 Polyurethane-based graft copolymer; prepared 3.8 analogouslyto DE 19948004 - B4 (page 27, example 2), solids content adjusted to32.5% by weight with water

Production of the Blue Paste:

The blue paste was produced from 69.8 parts by weight of an acrylatedpolyurethane dispersion produced as per international patent applicationWO 91/15528, binder dispersion A, 12.5 parts by weight of Paliogen® BlueL 6482, 1.5 parts by weight of dimethylethanolamine (10% indemineralized water), 1.2 parts by weight of a commercial polyether(Pluriol® P900 from BASF SE) and 15 parts by weight of deionized water.

Production of the Carbon Black Paste:

The carbon black paste was produced from 25 parts by weight of anacrylated polyurethane dispersion produced as per international patentapplication WO 91/15528, binder dispersion A, 10 parts by weight ofcarbon black, 0.1 part by weight of methyl isobutyl ketone, 1.36 partsby weight of dimethylethanolamine (10% in demineralized water), 2 partsby weight of a commercial polyether (Pluriol® P900 from BASF SE) and61.45 parts by weight of deionized water.

3. Preparation of a Non-Inventive Waterborne Basecoat Material 3

The waterborne basecoat material 3 was produced analogously to table B,except that, rather than the dispersion of the polyurethane-based graftcopolymer, prepared analogously to DE 19948004-B4 (page 27, example 2),the dispersion of a copolymer (CP) described in A) was used.

4. Preparation of a Non-Inventive Waterborne Basecoat Material 4

The waterborne basecoat material 4 was produced analogously to thewaterborne basecoat material 3, except that, rather than the polyesterprepared as per example D, column 16 lines 37-59 of DE-A-4009858, thereaction product (R) described in A) was used. The different amounts ofbutyl glycol in the waterborne basecoat material 4, caused by thedifferent solids content of the dispersion of the polyester fromDE-A-4009858 and the reaction product R, were compensated for byappropriate addition of butyl glycol.

5. Production of a Waterborne Basecoat Material I1 of the Invention

The waterborne basecoat material I1 was produced analogously to thewaterborne basecoat material 4, except that, in the aqueous phase,rather than 24.1% by weight of the dispersion of the copolymer (CP),only 16.5% by weight were used. In addition, 9.1% by weight of thepolyurethane resin (X) was used. The preparation of the polyurethaneresin (X) is guided by WO 92/15405, page 14 line 13 to page 15 line 13.The resin has an acid number of 25 mg KOH/g and a number-averagemolecular weight of 12 000 g/mol.

Comparison Between Waterborne Basecoat Materials 2-4 and I1

To determine the stone-chip resistance/stone-chip stability, multicoatpaint systems were produced by the following general method:

A cathodically electrocoated steel sheet of dimensions 10×20 cm servedas the substrate. First of all, the waterborne basecoat material 1 wasapplied pneumatically to this sheet in a film thickness of 18 to 22micrometres. After the basecoat material had been flashed off at roomtemperature for 4 min, waterborne basecoat materials 2-4 or I1 wereapplied, then flashed off at room temperature for 4 min, and thenintermediately dried in an air circulation oven at 70° C. for 10 min. Acustomary two-component clearcoat material was applied pneumatically ina film thickness of 35 to 45 micrometres to the intermediately driedwaterborne basecoat. The resulting clearcoat was flashed off at roomtemperature for 20 minutes. The waterborne basecoat and the clearcoatwere then cured in an air circulation oven at 160° C. for 30 minutes.

The multicoat paint systems thus obtained were examined for stone-chipresistance. For this purpose, the stone-chip test was conducted to DIN55966-1. The assessment of the results of the stone-chip test wasconducted to DIN EN ISO 20567-1. The results can be found in table 1.

TABLE 1 Stone-chip resistance of waterborne basecoat materials 2-4 andI1 on waterborne basecoat material 1 WBM Stone-chip result Assessment 23.5 not OK 3 3.0 not OK 4 2.0 not OK I1 1.5 OK

The results show that the use of a basecoat material (b.2.2.x) for usein accordance with the invention distinctly increases stone-chipresistance compared to waterborne basecoat materials 2-4.

6. Preparation of a Non-Inventive Waterborne Basecoat Material 5

The components listed under “aqueous phase” in table C were stirredtogether in the order stated to form an aqueous mixture. In the nextstep an organic mixture was prepared from the components listed under“organic phase”. The organic mixture was added to the aqueous mixture.The combined mixture was then stirred for 10 minutes and adjusted, usingdeionized water and dimethylethanolamine, to a pH of 8 and to a sprayviscosity of 58 mPas under a shearing load of 1000 s⁻¹ as measured witha rotary viscometer (Rheomat RM 180 instrument from Mettler-Toledo) at23° C.

TABLE C Waterborne basecoat material 5 Component Parts by weight Aqueousphase 3% Na—Mg sheet silicate solution 29.1 Deionized water 11.7 Butylglycol 4.1 Polyurethane-modified polyacrylate; prepared 2.8 as per page7 line 55 to page 8 line 23 of DE 4437535 A1 50% strength by weightsolution of Rheovis ® 0.2 PU 1250 (BASF), rheological agent Polyester;prepared as per example D, column 5.5 16 lines 37-59 of DE-A-4009858TMDD (BASF) 1.4 Melamine-formaldehyde resin (Luwipal 052 2.9 from BASFSE) 10% dimethylethanolamine in water 0.6 Polyurethane-based graftcopolymer; prepared 24.1 analogously to DE 19948004 - B4 (page 27,example 2), solids content adjusted to 32.5% by weight with waterIsopropanol 1.6 Isopar ® L from Exxon Mobil 2 Byk-347 ® from Altana 0.6Pluriol ® P 900 from BASF SE 0.4 Tinuvin ® 384-2 from BASF SE 0.7Tinuvin 123 from BASF SE 0.4 Carbon black paste 5 Blue paste 1 Micadispersion 5 Organic phase Aluminum pigment, available from Altana- 0.3Eckart Butyl glycol 0.3 Polyurethane-based graft copolymer; prepared 0.3analogously to DE 19948004 - 34 (page 27, example 2), solids contentadjusted to 32.5% by weight with water

Production of the Blue Paste:

The blue paste was produced from 69.8 parts by weight of an acrylatedpolyurethane dispersion produced as per international patent applicationWO 91/15528, binder dispersion A, 12.5 parts by weight of Paliogen® BlueL 6482, 1.5 parts by weight of dimethylethanolamine (10% indemineralized water), 1.2 parts by weight of a commercial polyether(Pluriol® P900 from BASF SE) and 15 parts by weight of deionized water.

Production of the Carbon Black Paste:

The carbon black paste was produced from 25 parts by weight of anacrylated polyurethane dispersion produced as per international patentapplication WO 91/15528, binder dispersion A, 10 parts by weight ofcarbon black, 0.1 part by weight of methyl isobutyl ketone, 1.36 partsby weight of dimethylethanolamine (10% in demineralized water), 2 partsby weight of a commercial polyether (Pluriol® P900 from BASF SE) and61.45 parts by weight of deionized water.

Production of the Mica Dispersion:

The mica dispersion was produced by mixing, using a stirrer unit, of 1.5parts by weight of polyurethane-based graft copolymer, preparedanalogously to DE 19948004-B4 (page 27, example 2), solids contentadjusted to 32.5% by weight with water, and 1.3 parts by weight of thecommercial mica Mearlin Ext. Fine Violet 539V from Merck.

7. Production of a Non-Inventive Waterborne Basecoat Material 6

The waterborne basecoat material 6 was produced analogously to table C,except that, rather than the dispersion of the polyurethane-based graftcopolymer, prepared analogously to DE 19948004-B4 (page 27, example 2),the dispersion of a copolymer (CP) described in A) was used.

8. Production of a Non-Inventive Waterborne Basecoat Material 7

The waterborne basecoat material 7 was produced analogously to thewaterborne basecoat material 6, except that, rather than the polyesterprepared as per example D, column 16 lines 37-59 of DE-A-4009858, thereaction product (R) described in A) was used. The different amounts ofbutyl glycol in the waterborne basecoat material 7, caused by thedifferent solids content of the dispersion of the polyester fromDE-A-4009858 and the reaction product R, were compensated for byappropriate addition of butyl glycol.

9. Production of a Waterborne Basecoat Material 12 of the Invention

The waterborne basecoat material 12 was produced analogously to thewaterborne basecoat material 7, except that, in the aqueous phase,rather than 24.1% by weight of the dispersion of the copolymer (CP),only 20.0% by weight were used. In addition, 6.5% by weight of thepolyurethane resin (X) was used. The preparation of the polyurethaneresin (X) is guided by WO 92/15405, page 14 line 13 to page 15 line 13.The resin has an acid number of 25 mg KOH/g and a number-averagemolecular weight of 12 000 g/mol.

Comparison of Waterborne Basecoat Materials 5-7 and 12

To determine the stone-chip resistance/stone-chip stability, multicoatpaint systems were produced by the following general method:

A cathodically electrocoated steel sheet of dimensions 10×20 cm servedas the substrate. First of all, the particular basecoat material wasapplied to this sheet pneumatically in a film thickness of 20 to 24micrometres. After the basecoat material had been flashed off at roomtemperature for 1 min, the basecoat material was intermediately dried inan air circulation oven at 70° C. for 10 min. A customary two-componentclearcoat material was applied pneumatically in a film thickness of 35to 45 micrometres to the intermediately dried waterborne basecoat. Theresulting clearcoat was flashed off at room temperature for 20 minutes.The waterborne basecoat and the clearcoat were then cured in an aircirculation oven at 160° C. for 30 minutes.

The multicoat paint systems thus obtained were examined for stone-chipresistance. For this purpose, the stone-chip test was conducted to DIN55966-1. The assessment of the results of the stone-chip test wasconducted to DIN EN ISO 20567-1. The results can be found in table 2.

TABLE 2 Stone-chip resistance of waterborne basecoat materials 5-7 andI2 WBM Stone-chip result Assessment 5 4.5 not OK 6 4.0 not OK 7 2.5 notOK I1 1.5 OK

The results show that the use of a basecoat material (b.2.1) distinctlyincreases stone-chip resistance compared to waterborne basecoatmaterials 5-7.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1:

Schematic formation of a multicoat paint system (M) of the invention,arranged on a metallic substrate (S), and comprising a cured electrocoat(E.1) and a basecoat (B.2.1) and a clearcoat (K), which have been curedjointly.

FIG. 2:

Schematic formation of a multicoat paint system (M) of the invention,arranged on a metallic substrate (S), and comprising a cured electrocoat(E.1), two basecoats (B.2.2.x), namely a first basecoat (B.2.2.a) and anuppermost basecoat (B.2.2.z) arranged above it, and a clearcoat (K),which have been cured jointly.

FIG. 3:

Schematic formation of a multicoat paint system (M) of the invention,arranged on a metallic substrate (S), and comprising a cured electrocoat(E.1), three basecoats (B.2.2.x), namely a first basecoat (B.2.2.a), abasecoat (B.2.2.b) arranged above it and an uppermost basecoat(B.2.2.z), and a clearcoat (K), which have been cured jointly.

1. A method for producing a multicoat paint system on a metallicsubstrate, comprising producing a cured electrocoat on the metallicsubstrate by electrophoretic application of an electrocoat to thesubstrate and subsequent curing of the electrocoat, producing a basecoator a plurality of directly successive basecoats directly on the curedelectrocoat by applying an aqueous basecoat material directly to theelectrocoat or applying a plurality of basecoat materials in directsuccession to the electrocoat, producing a clearcoat directly on thebasecoat or an uppermost basecoat by applying a clearcoat materialdirectly to the basecoat or the uppermost basecoat, jointly curing thebasecoat and the clearcoat or the basecoats and the clearcoat, whereinthe basecoat material or at least one of the basecoat materialscomprises the following components: at least one aqueous dispersioncomprising at least one copolymer (CP), said copolymer (CP) beingpreparable by (i) initially charging an aqueous dispersion of at leastone polyurethane, and then (ii) polymerizing a mixture of olefinicallyunsaturated monomers in the presence of the polyurethane from (i),wherein (a) a water-soluble initiator is used, (b) the olefinicallyunsaturated monomers are metered in such that a concentration of 6.0% byweight, based on the total amount of olefinically unsaturated monomersused for polymerization, in the reaction solution is not exceeded overthe entire reaction time, and (c) the mixture of the olefinicallyunsaturated monomers comprises at least one polyolefinically unsaturatedmonomer, at least one linear hydroxy-functional reaction product (R)having an acid number less than 20 mg KOH/g, the preparation of whichinvolves using at least one compound (v) containing two functionalgroups and an aliphatic or araliphatic hydrocarbyl radical which isarranged between the functional groups and has 12 to 70 carbon atoms,and at least one polyurethane resin (X), the preparation of whichinvolves using at least one compound (x.1) containing at least onecarboxylic acid group and at least one group reactive toward isocyanategroups.
 2. The method as claimed in claim 1, wherein the mixture ofolefinically unsaturated monomers comprises 0.1 to 6.0 mol % ofpolyolefinically unsaturated monomers.
 3. The method as claimed in claim1, wherein the mixture of olefinically unsaturated monomers comprisesallyl methacrylate, and no further polyolefinically unsaturated monomersare present.
 4. The method as claimed in claim 1, wherein dimeric fattyacids and/or dimer diols are used as compound (v) in the preparation ofthe reaction product.
 5. The method as claimed in claim 1, wherein thereaction product (R) is preparable by reaction of dimer fatty acids withaliphatic, araliphatic and/or aromatic dihydroxy-functional compoundshaving a number-average molecular weight of 120 to 6000 g/mol, thedihydroxy-functional compounds used being polyether diols, polyesterdiols and/or dimer diols.
 6. The method as claimed in claim 1, whereinthe polyurethane resin (X) is prepared using at least onealpha,alpha-dimethylolalkanoic acid as compound (x.1).
 7. The method asclaimed in claim 6, wherein the at least onealpha,alpha-dimethylolalkanoic acid is at least one selected from thegroup consisting of 2,2-dimethylolacetic acid, 2,2-dimethylolpropionicacid, 2,2-dimethylolbutyric acid and 2,2-dimethylolpentanoic acid. 8.The method as claimed in claim 1, wherein the basecoat material or atleast one of the basecoat materials additionally comprise at least onehydroxy-functional polymer other than (CP), (R) and (X) as a binder,selected from the group consisting of polyurethanes, polyesters,polyacrylates and copolymers of these polymers.
 9. The method as claimedin claim 1, wherein the basecoat material or at least one of thebasecoat materials additionally comprise a melamine resin as acrosslinking agent.
 10. The method as claimed in claim 1, wherein thebasecoat material or at least one of the basecoat materials comprise atleast one color pigment and/or effect pigment.
 11. The method as claimedin claim 1, wherein the basecoat material or at least one of thebasecoat materials is a one-component coating composition.
 12. Themethod as claimed in claim 1, wherein the joint curing is performed attemperatures of 100 to 250° C. for a period of 5 to 60 min.
 13. Themethod as claimed in claim 1, wherein the substrate used is anautomobile body.
 14. The method as claimed in claim 1, wherein themixture of olefinically unsaturated monomers comprises less than 10.0%by weight of vinylaromatic monomers, based on the total amount ofolefinically unsaturated monomers used for polymerization.
 15. Amulticoat paint system which has been produced by the method as claimedin claim
 1. 16. The method as claimed in claim 8, wherein all of thebasecoat materials additionally comprise at least one hydroxy-functionalpolymer other than (CP), (R) and (X) as a binder, selected from thegroup consisting of polyurethanes, polyesters, polyacrylates andcopolymers of these polymers.
 17. The method as claimed in claim 9,wherein all of the basecoat materials additionally comprise a melamineresin as a crosslinking agent.
 18. The method as claimed in claim 10,wherein all of the basecoat materials comprise at least one colorpigment and/or effect pigment.
 19. The method as claimed in claim 11,wherein all of the basecoat materials are one-component coatingcompositions.